Locking tongue for seat belt and seat belt device

A locking tongue for a seat belt of a vehicle according to one embodiment includes: a tongue body including a main wall and a pair of side walls; a clamp member slidable in a direction toward the main wall; a rotating member rotatably supported by the side walls; and an urging member that urges the clamp member in a direction away from the main wall. In a normal state, a gap larger than a thickness of a webbing that is the seat belt is formed between the clamp member and the main wall. In an emergency of the vehicle, tension of a lap belt becomes greater than tension of a shoulder belt, causing the rotating member to rotate in a direction from the shoulder belt toward the lap belt against urging force of the urging member, such that the webbing is clamped between the clamp member and the main wall.