An adapter lens has a front plate, an optical member, a side plate, an upper plate and a rear plate. The adapter lens can be detachably attached to an upper left-hand corner of a body included in a camera. The adapter lens is mounted on the camera with the front plate, the side plate, the upper plate and the rear plate in contact with a part of the front surface around the objective lens, an upper part of the side surface near the corner of the body, a part of the upper surface near the corner of the body, and an upper part of the rear surface near the corner of the body, respectively. A screw is passed through a through hole formed in connecting part of the rear plate and screwed into a threaded hole formed in a boss formed integrally with the side plate to hold the body between the front plate and the rear plate.