PURPOSE: To widen a focusing-position detectable range by photodetecting image forming luminous flux which differs in visual field from image forming luminous flux that a sensr photodetects by a sensor at optically different distance from an expected image formation surface.

CONSTITUTION: Photodetectors 4, 5, and 6 are arranged where luminous flux 2 centering on the optical axis of an image forming lens 1, and the 4th photodetector 20 is arranged more remotely from the expected image formation surface 3 than the photodetector 6, i.e. where luminous flux 19 differing in visual field from luminous flux 2 is received near the luminous flux 2. Namely, a light splitter 23 is provided where luminous flux reflected by a submirror 21 provided on the optical axis is received to split the incident luminous flux into three, which are guided to photodetectors 25-1, 25-2, and 25-3 (concrete photodetectors of photodetectors 4, 5, and 6 mentioned above); and the luminous flux is guided to a photodetector 25-4 (concrete one of photodetector 20 in said figure) like a CCD line sensor through a submirror 22.