Treatment instrument with movable handle and fixed handle includes biasing member for biasing the movable handle in an opening direction and an assist system with attraction force, e.g., magnetic force, to attract the movable handle in a closing direction toward the fixed handle. Range of movement of movable handle includes first movable range and second movable range. The second movable range occurs when a second gripping piece, e.g. a jaw, is in contact with respect to a first gripping piece, e.g., a vibration transmission member, and the movable handle is then further moved in a direction toward the fixed handle to a fully closed position. In the second movable range, attraction forces associated with the assist system are smaller than biasing force from the biasing member so that, upon release of a gripping force by an operator, the movable handle moves away from the fixed handle.