A mirror retention structure to reduce rebounding of a sub-mirror and to prevent shifting of the mirror-down position. The mirror retention structure includes, a main mirror retention frame that retains a main mirror, a sub-mirror retention frame that retains a sub-mirror and is axially supported at the main mirror retention frame, and a receiving pin that abuts against the sub-mirror retention frame and sets an angle of the sub-mirror at a mirror-down position, wherein the sub-mirror retention frame is provided with a first abutting portion that touches against the receiving pin at the mirror-down position, and a second abutting portion, at a different position from the first abutting portion, that touches against the receiving pin at a position prior to the mirror-down position in a transition from a mirror-up position to the mirror-down position.