PURPOSE: To provide a stable device of high economic efficiency by separating a position determining signal and a position detecting signal by using a rotary disc having no positioning holes and one piece of photodetector.

CONSTITUTION: A rotator 11 is formed with plural pieces of holes D in the positions equally dividing the circumference near its outside circumference and no hole is formed at one place. A photodetector 10 includes one piece of light source 13 and one piece of photodetector 15 and is dispposed opposite to the holes D with the disc 11 in-between. The output (a) of the photodetector 10 is applied via inverters 17, 19 to the clock terminal of a D type fli-flop 21. The D type FF21 operates as a monostable multivibrator together with a CR element. A D type FF27 operates similarly to the FF21 as well. The Q2 output signal and the output signal (c) of the inverter 19 are applied to the input terminal of an OR gate 33, and the output signal thereof is a position detecting signal VA. The output signal of the inverter 19 and the Q3 output signal of a monostable multivibrator 37 are applied to the input of an AND gate 39, and the output signal of the gate 39 is positioning signal VB.