A lens barrel cam mechanism including: a linearly movable member for holding an optical element and which can be moved in the optical axis direction of said optical element, said linearly movable member including a cam follower; and a rotary member rotable relative to said linearly movable member and which can be also moved relatively to said optical axis direction, said rotary member having a cam groove with which said am follower is slidably engaged formed on its circumferenclal surface, wherein said rotary member is rotated to guide said cam follower with said cam groove so that said linearly movable member is moved forward and backward in said optical axis direction, comprising: a gear portion provided at least at a part of one end of said rotary member, seen in said optical axis direction, said gear portion being developed at the outside of said end of said rotary member, seen in a plane perpendicular to said optical axis direction, and said gear portion having teeth on its outer periphery; a cam groove opening portion provided at an end portion of said cam groove characterized in that said gear portion bridges over said opening portion such that they become substantially coincident with each other on said plane being perpendicular to said optical axis direction and in that the opening extends on said plane towards said gear portion. a linearly movable member for holding an optical element and which can be moved in the optical axis direction of said optical element, said linearly movable member including a cam follower; and a rotary member rotable relative to said linearly movable member and which can be also moved relatively to said optical axis direction, said rotary member having a cam groove with which said am follower is slidably engaged formed on its circumferenclal surface, wherein said rotary member is rotated to guide said cam follower with said cam groove so that said linearly movable member is moved forward and backward in said optical axis direction, comprising: a gear portion provided at least at a part of one end of said rotary member, seen in said optical axis direction, said gear portion being developed at the outside of said end of said rotary member, seen in a plane perpendicular to said optical axis direction, and said gear portion having teeth on its outer periphery; a cam groove opening portion provided at an end portion of said cam groove characterized in that said gear portion bridges over said opening portion such that they become substantially coincident with each other on said plane being perpendicular to said optical axis direction and in that the opening extends on said plane towards said gear portion. a gear portion provided at least at a part of one end of said rotary member, seen in said optical axis direction, said gear portion being developed at the outside of said end of said rotary member, seen in a plane perpendicular to said optical axis direction, and said gear portion having teeth on its outer periphery; a cam groove opening portion provided at an end portion of said cam groove characterized in that said gear portion bridges over said opening portion such that they become substantially coincident with each other on said plane being perpendicular to said optical axis direction and in that the opening extends on said plane towards said gear portion.