PURPOSE: To detect focusing precisely with simple constitution by allowing a photoelectric converting element consisting of a light position detector to receive an object image formed of pieces of lumnous flux passed through different paths of an optical system, and detecting the relative position relation of the image.

CONSTITUTION: The light position detector consists of a high-resistance Si substrate 10, P type resistance laye 11, n-1 layer 12, and electrodes 13, 14 and 15, and when light L is incident to the P type resistance layer 11, currents IA and IB corresponding to the incidence position are obtained at the electrodes 14 and 15 by lateral photo effect to find the center of gravity of the image. An iris splitter 35 makes light transmission parts 35a, 35b, 35c and 35d transparent successively and a two-dimensional light position detector 37 detects a vertical and a horizontal direction through electrodes 38 and 39, and 40 and 41. Noises are removed through a filter of which a center frequency is equal to the operating frequency of the iris splitter 35 to obtain a composite output, i.e. the sum of absolute values or squares of respective components.