To provide an optical unit and a camera body which are configured so that the optical unit is easily connected and disconnected and can be attached to the camera body with high accuracy, to provide a connection structure for fastening an optical unit and a camera body, and to provide a camera.

In a mount mechanism 30 for connecting a lens barrel 20 to the camera body 10 so that the lens barrel 20 is detachable, the engagement pins 54, 54 of a lens mount 50 provided on the lens barrel 20 are fitted in the positioning holes 43 and 44 of a body mount 40 provided on the camera body 10. Thus, the lens barrel 20 is regulated in vertical and horizontal position with respect to the camera body 10 and connected thereto.