A light wave range finder has a range finding optical system in which the intensity of a coherent beam of light emitted from a coherent light source is modulated and delivered to an object and a reflecting beam of light reflected on the object is received by an light receiving element. A distance to the object is measured based on a phase detection of the reflecting beam of light. The range finding optical system has an optical substrate which is provided with, at least, a light intensity modulating portion for modulating the intensity of a coherent beam of light; an optical path changing portion for changing the coherent beam of light, which has been modulated the intensity by the light intensity modulating portion, between an external range finding optical path and a reference optical path; a first light amount regulating portion for regulating a light amount of the coherent beam of light guided to the external range finding optical path; a delivering system for delivering the coherent beam of light which has been regulated the light amount of the first light amount regulating portion toward an object; and a light receiving system for receiving a reflecting beam of light reflected by the object.