A rectilinear barrel of a camera supporting a lens group in a manner that the lens group is movable along a rectilinear slot in the direction of its optical axis, a cam ring which has on its circumference a rib with a cutout portion and which rotates to drive the lens group in the direction of the optical axis, a threaded driving shaft for moving the rectilinear barrel between an imaging position and a retracted position, a zoom gear for rotating the cam ring when the rectilinear barrel is at the imaging position, and a locking tab for allowing the rectilinear barrel to move by means of the threaded driving shaft by aligning itself with the cutout portion and for locking the movement of the cam ring, and thus, the rectilinear barrel by aligning itself with the rib.