PURPOSE: To reduce a movable part and to obtain a high-accuracy, compact device by composing the device of a projection and a photodetection lens and a projecting and a photodetecting element, and displacing the projection lens and photodetection lens which differ in focal length integrally.

CONSTITUTION: Light from the projecting element 1 passed through the projection lens 2 is reflected by a subject and incident on the photodetecting element 4 through the photodetection lens 3. A holder 21 which supports the lenses 2 and 3 which are different in focal length, but molded integrally is energized on a shaft 22 by a spring 23 to rotate freely clockwise and counterclockwise, and the elements 1 and 4 are fixed to a frame 20. One end 26 of a lever 24 supported pivotally on the shaft 25 is pressed against the holder 21, and the other end 26 is pressed against the lens barrel of the photographic lens 8; and the projection and photodetection systems move to right and left in one body associatively with the focusing operation of the lens 8. Therefore, the lever which rotates is provided at only one place and there is no relative movement between the projection and photodetection system, so there is not any influence of component working accuracy and high accuracy is obtained by the compact device.