PURPOSE: To accurately maintain the contents of memory for a long time and increase the life of a mirror position detector by digitally storing the position of a mirror in a memory circuit and employing a variable inductance of non- contact structure in a mirror position detector.

CONSTITUTION: A manual switch 30 is turned on and a mirror 29 is manually adjusted. Then, a memory switch 18 is turned on and a memory circuit 24 is actuated through pulses from a pulse generator 15. This memory circuit 24 stores the mirror position data of a circuit 17 which counts the pulses converted from the frequency of an oscillator 12, corresponding to the detected value for the mirror position of a variable inductance 11. Here, after adjusting the mirror 29 in a position suited to another driver, a return switch 21 is pressed to return the mirror 29 again to the stored position. Output data from the counter circuit 17 are compared with memory data from the memory circuit 24 in a comparison circuit 25, and the mirror 29 is rotated through a mirror drive circuit 28 until the both data agree with each other, returning the mirror 29 to the stored position.