Seat belt tightening device

An arresting hook plate (33) extending from a seatbelt buckle (34) is attached to a buckle plate (32) so that it moves freely along a straight line. When inertia force acts on the mass body inside a pull-in section casing, it leaves a lock state and is caused to move by the spring biasing force. The arresting hook plate (33) attached to the mass body by a wire (17) is caused to be pulled in along with movement of the mass body. A pair of locking pawls (36A,36B) included on the buckle plate prevent movement of the arresting hook plate (33) in a pull-out direction when they are in a locked position. There is also an locking pawl holder (40) which is used to select and set the locking or release of the locking pawls (362). If the locking of the locking pawls (362) is released so that the mass body is returned to the origin position against the spring (38) biasing force by means of the arresting hook plate (33) via the wire (17) , the mass body is placed again in a lock state due to the retainer (18).