1. Field of the Invention The present invention relates to a data reproduction method and a data reproduction apparatus which are used in a digital magnetic recording/reproducing system having a magnetic recording device for recording data on a magnetic recording medium such as a hard disk and a head having an output signal proportional to a magnetized state of the medium for reproducing the data. 2. Description of the Related Art A conventional digital magnetic recording/reproducing system, as shown in FIG. 9, includes a magnetic recording device 101 for writing data on a recording medium and a magnetic reproducing device 102 for reproducing the written data from the recording medium. The magnetic recording device 101 includes a magnetic head 103 mounted on a rotary drum 104 for writing data on the magnetic medium mounted on a disk 105 rotatably installed at a specific rotational speed. The reproducing device 102 includes a magnetic head 108 for reading the data from the disk 105, an amplifier 109 and an equalizer 112 having a band-pass characteristic for amplifying the magnetic signal read out of the magnetic head 108, a phase shifter (delay) 112, a filter 110 for reducing noise from the amplified magnetic signal from the amplifier 109, and a comparator 112 for comparing the signal read out of the magnetic head 108 with a slice level and outputting a reproduction signal. FIG. 10A is a graph of a recording signal and a recording frequency band generated on the disk 105. FIG. 10B is a graph showing the frequency characteristics of the equalizer 112 in the digital magnetic recording/reproducing system shown in FIG. 9. In a conventional digital magnetic recording/reproducing system, the frequency characteristics of the recording system for recording data on a magnetic disk (medium) 105 is as shown in FIG. 10B. As shown in FIG. 10A, at frequency a, the signal has an intense side lobe portion, and at frequencies b and c, the signal has a tail of an unnecessary signal. This unwanted signal is eliminated by the equalizer 112 of a band-pass characteristic. The band-pass frequency of the equalizer 112 is at Fc=c/(taut) and is fixed to about 30 kHz. The conventional data reproduction apparatus for the above-mentioned digital magnetic recording/reproducing system compares a signal read out of a recording medium by a magnetic head 108 with a slice level set at a center of an amplitude, as shown in FIG. 11, and detects a binary data value of 0 or 1. Therefore, even if the signal read out of the recording medium has side lobe portions at frequency a, b or c shown in FIG. 10A, the reproduced signal is compared with a slice level and then is binarized so that the reproduced data will not be affected by the side lobe portion. Thus, even if there are small interferences on the recording medium, the interferences can be prevented from affecting data reproduction. In the above-mentioned conventional apparatus, however, even when the reproduced signal has no side lobe portion, as shown in FIG. 12, the reproduced data can be affected by interferences produced by the recording medium because the equalizer frequency band-pass characteristic is narrow. As shown in FIG. 12, the side lobe portion at frequency a has the lowest intensity among the recording signals read out of the recording medium. Therefore, even if the slice level is set on the center of the reproduced signal shown in FIG. 12, the reproduced signal portion corresponding to frequency a is always detected as "1" because the signal level becomes higher than the slice level. As a result, if the reproduced signal portion corresponding to frequency a continues for several bits, the binary data of several bits after that are misrecognized as data representing a "1", and thus the data is erroneously reproduced. In order to solve the above problem, there has been proposed a reproducing method, for example, as shown in FIG. 13, in which data detection is not performed on all bits but only on bits at which there is no interferences to be provided as a result of the detection result on the data detection on the former bits. That is, in the case shown in FIG. 13, first the data detection is performed on the first and third bits, and the first and third bit data detected as "1" are detected as "1", and a bit of data detected as "0" is detected as "0". Second, the data detection is performed on the second bit, and the second bit data detected as "0" is detected as "1". In this manner, according to the above-mentioned conventional data detection method, the output bit data always has a transition at a boundary of "1" to "0", so that erroneous data detection will not occur. In this case, however, because of processing timing at which the data detection is performed on a magnetic head 108 shown in FIG. 11, the reproduced data changes at every 1 to 2 nsec. Therefore, it becomes necessary for the digital magnetic recording/reproducing system to output a reproduced data at a frequency of 10 to 20 MHz. If the frequency of the reproduced data becomes high, it becomes difficult to make use of the data in subsequent circuits, and if it is not possible, all the signals read out of the recording medium cannot be used for the reproducing operation. Therefore, in the above-mentioned conventional method, there has been a problem in that the frequency band of the reproduced data cannot be reduced beyond the above-mentioned limit. Furthermore, when the signals read out of the recording medium are amplified by the amplifier 109 shown in FIG. 9, the signal level may fluctuate as shown in FIG. 14, due to a temperature change of the disk 105. When the frequency characteristics of the equalizer 112 are as shown in FIG. 15, for example, the signal output by the equalizer 112 is affected by the amplified signal as shown in FIG. 14. FIG. 15 is a frequency characteristics of the equalizer 112 when a reproducing operation is performed at -40.degree. C., when a reproducing operation is performed at 0.degree. C. and when a reproducing operation is performed at 70.degree. C. The equalizer 112 is designed to have fixed output values a1, a2 and a3 at corresponding frequencies b1, b2 and b3 at which the signal fluctuates with the temperatures of the disk 105 shown in FIG. 14. In the conventional data reproduction apparatus, a