The present invention provides an intermediate adapter capable of being mounted on a first camera on which a first interchangeable lens is mounted and a second camera on which the first interchangeable lens and a second interchangeable lens are mounted. The second interchangeable lens includes a protruding portion protruding toward an image plane from its mount reference surface whose protruding amount is larger than that of the first interchangeable lens and has a flange back equal to that of the first interchangeable lens. The first camera has a first wall portion inside its mount and prevents mounting of the second interchangeable lens by the protruding portion of the second interchangeable lens contacting the first wall portion. The second camera includes a second wall portion provided inside its mount at a position retreated from the protruding portion of the second interchangeable lens.