1. Field of the Invention The present invention relates to a device to detect abnormality of an air fuel ratio sensor and to an abnormality diagnosis system for an air fuel ratio sensor. 2. Description of the Related Art A device to diagnose an output value of an air fuel ratio sensor has been proposed in which a change in output value of an air fuel ratio sensor is computed from a change in output value of a sensor for detecting the oxygen concentration in exhaust gas to thereby diagnose abnormality of the sensor. For example, as shown in FIG. 9, an abnormality diagnosis device for an air fuel ratio sensor has been proposed which includes an air fuel ratio sensor and a gas sensor which have individual output ranges for air-fuel mixture combustion, respectively, provided for an exhaust passage of an internal combustion engine. The abnormality diagnosis device includes change determining means, as means for computing the change in output value of the air fuel ratio sensor from the change in output value of the gas sensor, for determining a difference between output values of the air fuel ratio sensor and the gas sensor and change outputting means for outputting the difference, that is, the change in output value of the air fuel ratio sensor, to an external ECU. For example, a device has been proposed in which change outputting means has a comparator having a positive terminal connected to the gas sensor and a negative terminal connected to the air fuel ratio sensor, and determines the difference between the output value of the air fuel ratio sensor and the output value of the gas sensor based on an output value of the comparator and outputs the difference. In the abnormality diagnosis device for an air fuel ratio sensor, abnormality diagnosis is performed by computing the change in output value of the air fuel ratio sensor from the change in output value of the gas sensor. Therefore, even if an output value of the air fuel ratio sensor is saturated or if an error occurs in an output value of the gas sensor, abnormality diagnosis cannot be performed. Therefore, even if, for example, the air fuel ratio is lean (smaller than the theoretical air fuel ratio), abnormality diagnosis can be performed. However, in case where the air fuel ratio is, for example, rich (larger than the theoretical air fuel ratio), abnormality diagnosis cannot be performed. This is because the output value of the gas sensor is saturated. For example, when oxygen concentration detection is performed by an oxygen sensor, a change in voltage corresponding to the oxygen concentration is obtained in the form of a linear voltage. In addition, for example, when using a catalyst converter, which reduces NOx in the exhaust gas, in an exhaust passage of an internal combustion engine and which is provided with a plurality of exhaust gas sensors provided downstream of the catalyst converter in the exhaust passage, an amount of change in voltage corresponding to the oxygen concentration is obtained in the form of a step-like voltage waveform (pulse-like voltage waveform). FIGS. 10A and 10B are graphs each illustrating output values of an air fuel ratio sensor and an oxygen sensor and a change in output value of an air fuel ratio sensor from a change in output value of an oxygen sensor in a steady state. In the device disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2001-115057, an output value of an oxygen sensor (gas sensor) is used as a base, and a change in output value of an air fuel ratio sensor is determined from the change in output value of the oxygen sensor. For example, as shown in FIG. 10A, it is assumed that the output value of the oxygen sensor is “+”, the output value of the air fuel ratio sensor is “X”, and when the oxygen concentration increases from A (approximately 12%) to B (approximately 15%) as indicated by arrow {circle around (1)}, the output value of the air fuel ratio sensor is reduced from Xa (approximately 14.1) to Xb (approximately 10). In this case, the change in output value of the oxygen sensor is a step-like voltage waveform having an increased voltage. In this example, a change in output value of the oxygen sensor from A (approximately 12%) to B (approximately 15%) is the maximum. On the other hand, when the oxygen concentration decreases from A (approximately 12%) to C (approximately 6%) as indicated by arrow {circle around (2)}, the output value of the air fuel ratio sensor is reduced from Xa (approximately 14.1) to Xc (approximately 6). In this case, the change in output value of the oxygen sensor is a step-like voltage waveform having a reduced voltage. In this example, a change in output value of the oxygen sensor from A (approximately 12%) to C (approximately 6%) is the maximum. The output value of the air fuel ratio sensor and the output value of the oxygen sensor are linearly computed by using, as an intersection with an output axis, a location of the reduced voltage as a base. At the location of the reduced voltage, the output value of the oxygen sensor has changed from the maximum value to the minimum value, and at this time, the output value of the air fuel ratio sensor is changed. As described above, in the device disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2001-115057, when the output value of the air fuel ratio sensor has been changed in a range in which the output value of the gas sensor is saturated, for example, in a step-like change range which is obtained when an output value of the oxygen sensor is in a saturated state, an error occurs in a detection value. Therefore, abnormality cannot be detected. FIG. 10B shows examples in which output values of an air fuel ratio sensor and an oxygen sensor are changed from A (approximately 12%) to C (approximately 6%) and a change in output value of the oxygen sensor is saturated. As can be understood from FIG. 10B, the output value of the air fuel ratio sensor is reduced by Xc (approximately 6) to Xa (approximately 14.1) while a change in output value of the oxygen sensor is changed from Xa (approximately 14.1) to Xb (approximately 10) in the saturation range. Accordingly, an output value of the air fuel ratio sensor changes at a rate of Xc/Xa. On the other hand, an output value of the oxygen sensor decreases by only Xb (approximately 10), which is a constant value, from the maximum value. Therefore, when the output value of the air fuel ratio sensor is reduced by only Xb (approximately 10) from the maximum value in the saturation range of the oxygen sensor, a portion of the air fuel ratio sensor which should be reduced to a theoretical value of the air fuel ratio is reduced to a value smaller than the theoretical value of the air fuel ratio, and when the change in output value of the air fuel ratio sensor is computed by using the maximum value of the change in output value of the oxygen sensor, a change in output value of the air fuel ratio sensor becomes small. In a device disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2006-272433, at least one of an air fuel ratio sensor and an oxygen sensor is provided in each of a plurality of cylinders and a value of air fuel ratio of each cylinder is calculated. An output value of an air fuel ratio sensor is corrected to be equal to a theoretical value of an air fuel ratio by adding correction data and/or a difference in correction data obtained from the value of air fuel ratio of each cylinder to/from the theoretical value of the air fuel ratio. The air fuel ratio obtained by the correction can be used for abnormality diagnosis of an air fuel ratio sensor. When the air fuel ratio of the cylinder in which the output value of the air fuel ratio sensor is reduced from the theoretical value by only Xb (approximately 10) is corrected to be the theoretical value by adding correction data, for example, Xb (approximately 10) is set as a correction value corresponding to the output value of the air fuel ratio sensor in which the air fuel ratio is saturated. Therefore, when the output value of the air fuel ratio sensor changes from Xa (approximately 14.1) to Xb (approximately 10), an output value of the air fuel ratio sensor becomes equal to Xb (approximately 10). In this case, however, the output value of the air fuel ratio sensor changes in the same manner as in the case where the output value of the air fuel ratio sensor remains equal to Xb (approximately 10). In this case, when the output value of the air fuel ratio sensor is reduced by only Xb (approximately 10) from the theoretical value, abnormality is not accurately detected. In order to accurately detect abnormality of an air fuel ratio sensor, it is necessary to reduce the output value of the air fuel ratio sensor which is reduced from the theoretical value in the saturation range of the oxygen sensor to a value smaller than the theoretical value of the air fuel ratio. Therefore, it is considered that abnormality of an air fuel ratio sensor is not accurately detected when the output value of the air fuel ratio sensor is reduced by only Xb (approximately 10) from the theoretical value.