A collapsible lens mount optical unit comprises a fixed lens mount, a lens mount movable along the optical axis with respect to the fixed lens mount, an infrared ray intercepting filter interposed between the fixed lens mount and the lens mount. The collapsible lens mount optical unit further comprises a move-aside mechanism. When the lens mount is retracted and made to approach the infrared ray intercepting filter, the move-aside mechanism moves aside the infrared intercepting filter out of the optical axis in a direction perpendicular to the optical axis. The lens can be accommodated in a position on the optical axis after the infrared intercepting filter is moved.