A lens apparatus which is easy to assemble, provides high strength at the flange portion, and achieves high degree of design freedom, too, is disclosed. The lens apparatus comprises first and second members. The first member has a guide portion to guide the member straight ahead in an optical axis direction. The second member is arranged outside the first member and has a circumferential wall, on which an engaging portion for guiding the guide portion is formed. An opening portion is also formed at the end of the optical axis direction of the second member at a different position from the engaging portion and connected with the engaging portion. The opening portion takes the guide portion in and out with respect to the second member.