PURPOSE: To eliminate erroneous speed detection when a rotary direction is changed over, by detecting the change in a rotary direction and removing the speed detection signal at this detection time as an error from a speed detection signal line.

CONSTITUTION: A rotary speed detector 2 resets counting every when a signal RST is imparted and performs counting on the basis of the signals fA, fB from a two-phase pulse pick-up 1 or a combination thereof to output a counted value directly before the signal RST as a speed detection signal N. When there is change in a rotary direction during counting operation, a FF circuit 8 is set by the outputs of direction detectors 3, 4 and the error of the speed detection signal is transmitted to the side of a control apparatus main body by the output of said circuit 8 while the control apparatus main body neglects the speed detection signal.