PURPOSE: To increase the differences in optical path length and to determine focusing accurately by allowing imaging luminous fluxes to transmit through an optical splitter to conduct the same to the 1st photodetector, reflecting the fluxes with the 1st splitter to conduct part thereof to the 2nd beam splitter, and conducting part thereof to the 3rd beam splitter and the reflection surface thereby forming three optical paths having differences in optical path length.

CONSTITUTION: The 1st beam splitter 11a of a luminous flux splitter 10 is formed between prisms 12a and 12b, the 2nd beam splitter 11b is interposed between the prisms 12b and 12c, the 3rd beam splitter 11c is interposed between the 1st and the 2nd prisms 12c and 12d, and the splitter 11c is provided so as to make 90° angle with the splitters 11a, 11b. The luminous flux la transmitted through the splitter 11a is made incident to a photodetector 16a. The luminous flux lb reflected by the splitter 11b is made incident to a photodetector 16b. The luminous flux transmitted through the splitter 11b is reflected by a total reflection surface 13 and is partly made incident to a photodetector 16c.