To solve the problem in focus detection in a phase difference detection system that it is difficult to carry out the focus detection of a wide viewing field range with high accuracy.

A sub-mirror (second optical element) 16 arranged above a main mirror (first optical element) 14 reflects the luminous flux reflected by the main mirror to a transverse direction intersecting with the optical axis of an imaging optical system and the focus is detected by a focus detecting optical system 26 based on the luminous flux reflected by the sub-mirror. The focus detecting optical system 26 is arranged in the side of the sub-mirror such that the focus is detected from the luminous flux reflected by the sub-mirror 16 in the transverse direction intersecting with the optical axis 13 of the imaging optical system.