When a cam barrel 24 is rotated on an outer periphery of a fixed barrel 23 during varying of power, a lens holding frame 22 that has cam followers 27, which engage with cam portions 29 provided in the cam barrel 24 and forward movement guide openings 28 provided in the fixed barrel 23, is moved straight forward in an optical axis direction by a cooperation between the cam portions 29 and the forward movement guide openings 28. A projection portion 30 is provided in an outer periphery of the diaphragm mechanism 15. A diaphragm aperture diameter is changed by rotating the projection portion 30 around an optical axis. The projection portion 30 engages with a diaphragm control cam member 31 fixed to an inner surface of the fixed barrel 23, and changes the diaphragm aperture diameter in response to movement of the lens holding frame 22.