Rotation detecting device

A rotation detecting device for detecting a rotation number and a rotation direction of a rotating member based on at least two detection signals having different phases from each other and output from at least two sensor elements provided on the rotating member so as to face each other characterized in that the rotation detecting device includes a waveform processing means for performing a waveform processing on the detection signals based on one of a plurality of predetermined conversion rules, a setting signal receiving means for receiving a setting signal for determining the predetermined conversion rule to be used from the plurality of predetermined conversion rules, and a conversion rule setting means for determining the predetermined conversion rule to be used based on the setting signal received by the setting signal receiving means.