1. A collapsible lens barrel used for an imaging device, the collapsible lens barrel comprising:a base barrel mounted to the imaging device;
a drive device including a linear motion member reciprocating in an optical axis direction and a drive source for driving the linear motion member;
a cam frame disposed inside the base barrel and provided with a cam groove;
a drive device frame that is engaged with the cam groove in the cam frame and causes the drive device to reciprocate in the optical axis direction;
a focus lens group frame that holds a focus lens group; and
a contact part and an arm that extend from the focus lens group frame toward an object side and are engaged with the linear motion member to cause the focus lens group frame to reciprocate in the optical axis direction,
wherein, in a collapsed position, a plurality of lens groups is disposed inside the arm.