PURPOSE: To obtain a device which causes no loss of a light quantity and has a good detecting sensitivity by a simple constitution, by constituting a photodetector of an automatic focusing device, of four photodetectors divided into four in axis symmetry, and installing thin prisms in front ot the photodetector, so as to cross by placing an optical axis between them.

CONSTITUTION: A photodetector 27 is constituted of four photodetectors 27a, 27b, 27c and 27d divided into four in axis symmetry, placed so as to be orthogonal to an optical axis of an optical system, and thin prisms 25, 26 are crossed and installed in front of said photodetector so as to become an axis symmetry by placing the optical axis between them. A light from a light emitting element 1 is reflected by a half mirror 2, transmits through an objective lens 3 and irradiates an object surface 4. The light reflected by the object surface 4 is converged by the objective lens 3, transmits through the half mirror 2, also transmits through the prisms 25, 26, and forms an image of two spots S1, S2 on the photodetector 27. The focusing of a photographic optical system is controlled by a signal detected by the photodetector 27.