1. Field of the Invention The present invention relates to an electric power steering device. 2. Description of the Related Art Japanese Patent No. 3735208 discloses a structure in which a rotation sensor for detecting steering torque is installed on a steering shaft, and the rotational angular velocity (rotational speed) of the rotation sensor is multiplied by an assist torque angle correction value calculated on the basis of steering torque information detected by the rotation sensor to correct the assist torque angle. Japanese Patent Laid-open No. 2011-239980 discloses a technology of measuring torque fluctuation amount of steering torque obtained by differentiating torque detection value from a steering operation start point to a steering operation end point, and detecting torque fluctuation amount when steering torque is 0 and removing the same as an abnormality in accordance with an upper limit value of the torque fluctuation amount. There is a case in which there is a difference between a detected value of rotational angle sensor and a real rotational angle due to variation of a motor and an actuator. This difference is detected as torque fluctuation. Therefore, according to Japanese Patent No. 3735208, if an abnormality has occurred in torque detection, abnormality of torque fluctuation is not accurately judged. According to Japanese Patent Laid-open No. 2011-239980, if the detected torque fluctuation amount exceeds an allowable range, abnormality of torque fluctuation is not detected. The present invention was made in view of such a circumstance, and aims to provide a steering device capable of correctly detecting abnormality of torque fluctuation caused in a torque detection device, or abnormal torque fluctuation.