an insulating member 45 to constitute the aforementioned switch 37 upon insertion of the mount 4 of the lens tube into the mount 34 of the camera body a head portion 382 of said pin 38 is pressed towards the right to the illustrated position by an annular projection 4b of the bayonet mount 4 whereby the end 381 of the pin 38 pushes an insulating sheet 46 to separate the movable contactor 371 from the fixed contactor 372 thereby opening the switch 37 in this state as shown by broken lines in fig5 a recess 4a in the lens mount is spaced by an angle u03b8 from the pin head 382 a subsequent anticlockwise rotation of the lens tube by the angle u03b8 bring the contacts 8a 8f on the lens to the full lined positions thus making respective connections with the contacts 36a 36f on the camera simultaneously the mount recess 4a is rotated anticlockwise from the broken lined position to the full lined position whereby the head 382 of the pin 38 engages with the recess 4a by the function of the compression spring thus completing the mounting operation of the interchangeable lens on the camera body at the same time the end 381 of said pin 38 moves to the left in.