A movement assist device includes a retraction tool having a case, a latch switched between a standby position which is locked inside the case and a retraction position which releases the locking so as to be capable of sliding, and an urging device; and a striker switching the latch from the standby position to the retraction position, or vise versa. The retraction tool or the striker is provided in a fixed body, or a moving body movably disposed in the fixed body. When the latch is switched from the standby position to the retraction position, a sliding door moves by an urging force. The case includes a striker leading portion notched such that a width of the case becomes thin to a middle of the other end from one end in a longitudinal direction. Also, one portion of the latch is exposed and disposed in the leading portion.