PURPOSE: To eliminate aberration and to obtain a small-sized focus detector with high accuracy by determining the angles of inclination of respective concave mirrors at the values lower then the value wherein the incident angles of the ray from the center of a region for detecting a primary image to the centers of a pair of the concave mirrors are expressed respectively by the specific relations with respect to the max. diameter of the concave mirrors.

CONSTITUTION: The incident angle of the ray from the center in the region 2A for detecting a primary image of a photographic lens 1 to the centers of a pair of concave mirrors 8A, 8B is so set as to attain the values lower than the values which the in the unit of radian is respectively expressed by the equations wherein the max. diameter of the respective concave faces 8A, 8B is defined as R. The imaging free from various aberrations is made possible in the regions 9A, 9B for detecting the planes of the secondary images formed by passing the light transmission parts 163c, 163b of a reflection material 163 by a pair of concave mirrors 164, 165 to the photoelectric element arrays 167, 169 in a lower block provided with a light shielding plate 162, a slit 162a (the above-described region 2A for detecting the primary image) and a lens 161 in the bottom of a camera, whereby the incorporation of the detector as a small- sized focus detector having excellent performance in the camera is made possible.