PURPOSE: To suppress a false signal and to make automatic focusing control accurate by arranging a low-pass filter in an optical system for focusing detection.

CONSTITUTION: A 1st mirror 3 is arranged as a luminous flux splitting element nearby the stop 4 of the photographic optical system consisting of a focusing lens group 1, a zoom lens group 2, and a relay lens group 5, and part of image formation luminous flux is guided to the optical system 8 for focus detection by a 2nd mirror 7 to form an image nearby a focus detecting element 9. Here, the 1st mirror 3 or 2nd mirror 7 has an optical low-pass filter on its surface. Thus, a frequency component which exceeds the spatial frequency detection limit of the focus detecting element is removed optically from the image formed by the optical system for focus detection to obtain an automatic focusing device which is prevented from malfunctioning.