In the normal rotation direction, a change in the main sensing signal caused by a front edge is defined as a signal change caused by an effective edge, and a change in the main sensing signal caused by a back edge is defined as a signal change caused by an ineffective edge. In the reverse direction, a change in the main sensing signal caused by the back edge is defined as a signal change caused by an effective edge, and a change caused by a front edge is defined as a signal change caused by an ineffective edge. Regardless of the rotation direction, a detection signal generating circuit generates a detection signal including falling-edge changes and rising edge changes caused by the effective edge and ineffective edge respectively. When the direction is changed, the signal change on the detection signal is prohibited. As a result, gear tooth detection discrepancies are prevented.