To wear and remove an assist instrument has been difficult because a user has to wear or remove the assist instrument on or from her/his body in a state that an elastic member is contracted. An assist instrument includes a wearing-removal member that is placed wearably and removably on a body of a user; an assist unit that is provided to the wearing-removal member, and assists muscular strength of the user by an action of at least one of extension and contraction by power supply; a control unit that controls an action of the assist unit; and something that adjusts constriction of the body by the wearing-removal member at a time of at least one of wearing and detachment of the wearing-removal member.