A movable member moved forward to a predetermined position can automatically move further forward. The mechanism includes first and second connecting members provided in the movable member and an immovable member. The second connecting member includes a catcher for catching the first connecting member and an urging device for urging the catcher. The catcher is arranged to be able to move or relatively move along a moving direction of the movable member, and held in a waiting position while receiving an urging force by the urging device toward a forward movement direction or backward movement direction of the movable member. When the movable member is moved to the predetermined position, the first connecting member is retained by the catcher of the second connecting member, and upon releasing of retention, the movable member is automatically moved forward to the stopped position by the urging force.