PURPOSE: To prevent mistaken detection of a direction of rotation by a noise element, by judging the detected direction of rotation as correct when the rotating direction stays unchanged during at least one period of a pulse-series signal from an rpm-detecting means.

CONSTITUTION: Pulse-series signals PG1, PG2 provided with a phase difference of 90° are supplied to clock- and data-terminals of a D-FF4 respectively from pulse generators 2, 3 accompanying revolution of a motor 1. Consequently, the value 1 of the signal PG2 representing the leading point of the signal PG1 is latched from the output terminal (Q) of the FF4 to be issued out free-running counter 5 and memory 9 as an output signal S1. The counter 5 counts time from one change of the signal S1 to another and the counter 7 counts time of one period of the signal PG1. A comparator 8 gives an output instruction to a memory 9 only when time between changes of the signal S1 is longer than the one period portion of the signal PG1. Consequently, the memory 9 issues out the signal S1 as a direction of rotation identifying signal S2.