To provide an attaching mechanism of a lens cap covering a lens unit for exchange on the lens mounting side in an image-pickup device such as an infrared camera.

As to this attaching mechanism for lens cap 1 constituted of a lens unit which is freely attachably/detachably attached to the image pickup device and also whose protruding part is projectingly set on the outside peripheral surface of the lens mount, and the lens cap 1 which covers the lens mount surface of the lens unit and in which protruding part 2 capable of being freely engaged/disengaged with the protruding part of the lens mount is projectingly set on an inside peripheral surface; the cap 1 is formed of an elastic member, also a slit 3 which is long in an inside peripheral direction is transmissively set apart behind the protruding part 2 projectingly set on the inside peripheral surface of the cap 1, and also a projecting part 4 is formed on a side wall peripheral edge part opposed to the slit 3. Thus, even though the position of the protruding part of the lens mount has variation in dimension, the lens cap can be surely attached. Furthermore, one lens cap can be attached and used in plural mounts.