a light measuring device for a single lens reflex camera wherein a reflection member having a slant surface is provided at the rear of an exit plane of a pentagonal roof prism and locating to be outside of the viewfinder light path , and the effect by undesired incident light is eliminated by a mirror formed by the slant surface placed in front of the light receiving element or photosensitive device , which light once enters into the pentagonal prism from the ocular lens and is reflected at the bottom surface thus again comes out of the surface of the pentagonal prism facing the ocular lens to proceed toward the photosensitive device .