PURPOSE: To reduce the necessity of the compensation of chromatic aberration and to simplify an optical system by forming the image of luminous flux extracted from a light dividing element on a focus detecting element through a concave reflector.

CONSTITUTION: The half mirror surface of the light dividing element H is formed as the concave reflector. If it is defined that an intersected point between the reflected surface of the light dividing element H and the optical axis of an optical image pickup system is an original point, a direction turned to the image surface of an optical axis is the (z) axis, the arraying direction of focus detecting elements is the (x) axis and the necessary focus distance of an auxiliary optical system arranged after an afocal part is (f) when a focus detecting element S is arranged on the (y) axis of the rectangular coordinate system, the shape of the reflecting surface becomes a spheric surface having 21/2nf radius around the spherical center P on the coordinates (0, n.f, -n.f). When the length from the original point of a dividing prism to the border surface in the (y) axis direction is defined as (d), the focal position where the focus detecting element S is arranged becomes (0, f-d/n, 0). In addition, the whole optical system can be reduced at its size without dropping its detecting accuracy by using a thin dividing prism H and forming the half mirror part HM like a square.