Lens barrel

This application claims benefit of Japanese Application No. 2001-154371 filed on May 23, 2001, the contents of which are incorporated herein by reference. An external helicoid thread is provided which includes a helicoid thread having a first ridge whose both flanks are formed at a first flank angle, and a helicoid thread having a second ridge whose both flanks are formed at a second flank angle which is different from the first flank angle. An internal helicoid thread is kept in screwed-state engagement with the external helicoid threads. And in a lens barrel obtained using a connection based on such internal and external helicoid threads, the connection provided by these helicoid threads can remain secured even if a sudden outside force is applied to the lens barrel.