A counterweight that biases a ratchet lever toward an unlocking side, in addition to a self weight of the ratchet lever, is provided in a mechanism that locks pull-out operation of a seatbelt according to operation of an acceleration sensor. The counterweight is provided with a moment transmission projection that allows the ratchet lever to turn freely as a result of non-engagement between the counterweight and the ratchet lever when a tilt angle of a seat back is less than 60°, and that transmits rotational moment by a self weight of the counterweight (rotational moment in the direction of an arrow R3) to the ratchet lever as a rotary bias force toward the unlocking side as a result of engagement between the counterweight and an engagement wall of the ratchet lever when the tilt angle of the seat back is 60° or more.