PURPOSE: To make the movement extent of a lens system constant in focusing operation for the same subject without reference to whether a main optical system is in a standard photographic or telephoto state and to simplify a mechanism by using the same main optical system for the focusing in both standard photographic and telephoto states.

CONSTITUTION: A movable mirror 4 is put off the effective luminous flux of the main optical system 1 and a shutter 6-1 is closed in the standard photographic state. Luminous flux from the subject forms an image directly on a photosensitive surface 7 through the main optical system 1 and the focusing is performed by moving the main optical system 1 on an optical axis S1. In the telephoto state, on the other hand, the main optical system 1 is moved on the optical axis S1 as shown by an arrow 11 and matched with an optical axis S2 in front of an auxiliary optical system 2 as shown by an arrow 12 and the movable mirror 4 is rotated around a rotating shaft 4A until a center light beam from a fixed mirror 3 is incident on the photosensitive surface 7 at a specific position, closing the shutter 6-2. Then, the focusing is carried out by moving the main optical system 1 on the optical axis S2.