To provide an extending cam mechanism for a zoom lens barrel, which not only secures the extent of movement of lenses and movement precision but also makes a cam ring small-sized.

In the extending cam mechanism for the zoom lens barrel, a plurality cam grooves having the same basic locus are formed in the cam ring in positions different in the direction of an optical axis and in the circumferential direction, and a plurality of cam followers to be engaged with the plurality of cam grooves respectively are formed in a linear motion ring in positions different in the direction of the optical axis and in the circumferential direction.