PURPOSE: To obtain a photodetector for spot photometry which photodetects the incident light to a relatively narrow specific part in an exposure image plane to measure the brightness or intensity of subject light passed through the photographic lens of a single-lens reflex camera.

CONSTITUTION: A flat-plate constituent 12 arranged under a focal plate 9 which constitutes the viewfinder optical system of the single-lens reflex camera has the 1st cut surface 14 slanting at 45° to an optical axis, and a translucent specular film is formed on the 1st cut surface 14 over the entire surface or to specific length; and part of the subject light from a reflecting mirror 8 passes through the cut surface 14 and travels straight toward the focal plate 9, and the remainder is reflected by the cut surface 14 and travels right in a figure. The 2nd cut surface 15 parallel to the optical axis 13, on the other hand, is provided at a position equal to the distance from an intersection to a focal plane 17 on the focal plate 9, and an amorphous photodiode layer 16 is formed at the position to desired area. Therefore, the photodiode 16 is positioned equivalently to the focal plane, and the subject light incident to the center part of the focal plate 9 is photodetected by the photodiode 16 at a position equivalent to the focal plane.