PURPOSE: To measure a distance with high accuracy by a method wherein a light- quantity gradient in a distance measuring light flux generated in a variable-density filter means is corrected by a corrective filter means having an optical density changing consecutively.

CONSTITUTION: The light of a light source 8 is switched over to a distance measuring light for a prism 6 and to a reference light for a mirror 14 alternately by an optical path switching unit 9 and the distance measuring light is reflected by a prism 6 and a dichroic prism 5 and irradiates a target substance through a lens l. The reflected light passes through the lens l and it is reflected by prisms 5 and 7 and reaches a photodetector 10 through two density filters 12 and 11. The reference light for the mirror 14 reaches the photodetector 10 through the filter 11. The filter 11 has an optical density changing along a circumferential direction, and when the quantities of the reference light and the distance measuring light are different, the quantities of the two lights are made to coincide with each other by rotating the filter 11. The density gradient of the filter 12 is the same as that of the filter 11, but reverse in the direction thereof, and a light-quantity gradient generated in the filter 11 is corrected by the filter 12. Thereby a measuring error due to the light-quantity gradient can be avoided.