A light receiving optical system includes: a relay optical system for converging light to be measured which has been converged on an image plane of an objective optical system. The light receiving optical system has a relay optical system with a relay lens operable to be selectively switched between first and a second conjugate positions, and a first and a second light flux limiting aperture members selectively switched between the first and the second conjugate positions. The relay optical system selectively forms, on the image plane of the objective optical system, an enlarged image and a reduced image of a view angle defining aperture in the case where the relay lens is selectively switched between the first and the second conjugate positions to define an incident light flux through the view angle defining aperture by a first and a second light flux limiting apertures, respectively.