PURPOSE: To focus return light to a fixed position in a ranging device for scanning irradiation light in multidirections to measure a distance from a plurality of targets.

CONSTITUTION: Irradiation light is reflected by one surface of a rotating a polygon mirror 3 and focused by a focus lens 4 and then applied to a target, the range of which is to be found. Return light reflected by a target, the range of which is to be found, is reflected by another surface of the polygon mirror 3 and then focused to a light-intercepting element 2. The focal point of return light, which mas varied in the past, can be fixed to one point, so that the area of a light-intercepting element may be reduced. Thereby, the response of the light-intercepting element itself can be increased. At the same time, since light giving disturbance to the light-intercepting element is not received, sensitivity can be increased.