PURPOSE: To prevent a lens holding member from coming off from a supporting member against impact force by providing an extended part which is extended in an optical axis direction in order to increase the fitting length of the lens holding member and the supporting member.

CONSTITUTION: Backward extended parts 1c extended backward in the optical axis direction are formed equally distributing at three position in the back end part of a 1st lens barrel 1. A recessed part to prevent interference with the extended part 1c is formed in respective 2nd and 3rd group units 2. Then, the fitting length of a straight advance guide barrel 9 to the lens barrel 1 in a telephoto state is L. Meanwhile, the fitting length thereof at the part where the extended part 1c is formed is (L+α), so that the fitting length is increased by α, that is, the length of the extended part 1c. Thus, even when the impact force that the leading tapered part of the lens barrel 1 is made to come off from the cam groove 6b of a male helicoid barrel 6 because of shaking and hitting or dropping acts on the lens barrel 1, the leading tapered part is prevented from coming off.