Abstract: A light measuring device for use in a single lens reflex camera includes: a transparent block located in the rear and upward location of a pentagonal prism and having at least one surface serving as a reflecting surface; and a light receiving portion including a photoelectric element having a light receiving surface, which portion is located in the vicinity of such one surface of the prism, such that part of the scene light which as been transmitted through an objective lens and scattered through the front portion of a focussing plate located in the optical path of a viewfinder is introduced into the transparent block, then reflected on such reflecting surface and introduced to the light receiving surface of the photoelectric element, while part of the scene light scattering through the rear portion of the focussing plate is transmitted through the transparent block and is then introduced into the light receiving surface, whereby the light beams from the entire surface of the scene may be measured at a desire