PURPOSE: To improve the reliability and safety by constructing such that the projection and notch of first and second disc rotatable integrally with the movable section rotary shaft are detected simultaneously thus to operate/output a position detecting signal.

CONSTITUTION: Discs 2, 3 will rotate together with a spindle 1 to detect a projection 2a and a notch 3a through proximity switches 4, 5. In other word, upon arriving of the projection 2a or notch 3a immediately above the spindle 1, detection signals 1, 0 are fed through an amplifier 6a and an inverter 6b to AND gate 6c. AND gate 6c will obtained the theoretical sum from said signals to produce position signal D3 as 1. With such arrangement, the position is detected through the other disc upon malfunction of one disc, resulting in improvement of the reliability and safety.