PURPOSE: To reduce mechanical vibration and to make adjustment for an electric circuit easy and to lower the cost by preparing a device giving a different optical path length by varying rotation in the forward direction of photoelectric element.

CONSTITUTION: On a disc like holder 2 parallel plane plate glass 1aW1e different in thickness and a hole 1f are placed and rotated with a motor 3, thereby allowing optical path length of light incident upon an image pick-up elememt 5 located on an optical axis to be changed. A picture signal from an image pick-up element 21 that receive a signal from a timing signal generating circuit 22 is outputted and passes through a band-pass filter 15, an absolute value circuit 26, an integrating circuit 27, a sample and hold circuit 28, a multiplexer 29 and an AD converter 30, thereby allowing a focal point position signal to be inputted to a microcomputer 31. The computer 31 receives a signal for arranging the optical path length via a photointerrupter 7 and a circuit 32, permitting a focal position adjusting signal to be sent to a driving circuit 33 of a Z axis driving motor 24. Through the process, a mechanical vibration is reduced and a circuit is adjusted easily and is lowered the cost.