To absorb backlash between a cam pin and a cam groove by using a pressing member simple in constitution and capable of being easily assembled by extending the pressing member between a lens frame and a nonrotating member and making the cam pin abut on one side wall of the can groove.

The cam pin 17 is fixed to a second lens frame 15 by a set screw 18 through a long hole opened at a sliding frame 7. The cam groove 3c is provided at the inner periphery of a cam barrel 3 together with a female helicoid 3b, and the cam pin 17 is engaged with the cam groove 3c. A tension spring 21 is hung between the rear end part 15b of the second lens frame 15 and a light shielding plate 22 that is the nonrotating member stuck orthogonally to an optical axis K at the rear end part of the sliding frame 7 positioned on the side of an image formation surface from the second lens frame 15, and the cam pin 17 is pressed backward together with the second lens frame 15. Thus, the cam pin 17 abuts only on the rear side wall of the cam groove 3c, so that the backlash in a direction in parallel with the optical axis K is solved.