A tongue slidably supported by a seat belt, the tongue partitioning the seat belt into a shoulder belt and a lap belt, the tongue capable of being engaged with a buckle, the tongue comprising: an elongate seat belt insertion hole configured to allow the seat belt to be inserted therethrough; and a seat belt sliding unit configured to form part of the seat belt insertion hole, the seat belt sliding unit including a seat belt sliding surface that allows the seat belt to slide thereon, wherein the seat belt sliding unit includes a predetermined number of concave portions (19, 20) in the seat belt sliding surface and a predetermined number of protrusions independently disposed in the concave portions (19, 20), wherein each of the predetermined number of protrusions has a substantially triangular cross section in a direction that is perpendicular or substantially perpendicular to the seat belt insertion hole, and each of the protrusions includes a flat or substantially flat end surface (25a1-25d1) located on the shoulder belt side and a flat or substantially flat end (25a2-25d2) located on the lap side, and wherein for at least some of the predetermined number of protrusions, an acute angle formed by an imaginary line (27, 31, 34) drawn from the topmost end of the protrusion to an arc center of an outer peripheral surface of the concave portion (19, 20) and the end surface of the protrusion on the shoulder belt side is less than an acute angle formed by the imaginary line (27, 31, 34) and the end surface on the lap belt side, characterized in that the top end at which the end surface of the protrusion on the shoulder belt side intersects with the end surface on the lap belt side is formed in an arc shape, and the seat belt sliding surface includes at least a shoulder belt side seat belt sliding surface extending in the length direction of the seat belt insertion hole and disposed on the shoulder belt side, wherein the shoulder belt side seat belt sliding surface extends from a shoulder belt side of a position at which a distance in a direction perpendicular or substantially perpendicular to the length direction of the seat belt insertion hole is minimized to a lap belt side of the position, and a portion of the shoulder belt side seat belt sliding surface is located on the lap belt side of the position at which the distance is minimized. an elongate seat belt insertion hole configured to allow the seat belt to be inserted therethrough; and a seat belt sliding unit configured to form part of the seat belt insertion hole, the seat belt sliding unit including a seat belt sliding surface that allows the seat belt to slide thereon, wherein the seat belt sliding unit includes a predetermined number of concave portions (19, 20) in the seat belt sliding surface and a predetermined number of protrusions independently disposed in the concave portions (19, 20), wherein each of the predetermined number of protrusions has a substantially triangular cross section in a direction that is perpendicular or substantially perpendicular to the seat belt insertion hole, and each of the protrusions includes a flat or substantially flat end surface (25a1-25d1) located on the shoulder belt side and a flat or substantially flat end (25a2-25d2) located on the lap side, and wherein for at least some of the predetermined number of protrusions, an acute angle formed by an imaginary line (27, 31, 34) drawn from the topmost end of the protrusion to an arc center of an outer peripheral surface of the concave portion (19, 20) and the end surface of the protrusion on the shoulder belt side is less than an acute angle formed by the imaginary line (27, 31, 34) and the end surface on the lap belt side, characterized in that the top end at which the end surface of the protrusion on the shoulder belt side intersects with the end surface on the lap belt side is formed in an arc shape, and the seat belt sliding surface includes at least a shoulder belt side seat belt sliding surface extending in the length direction of the seat belt insertion hole and disposed on the shoulder belt side, wherein the shoulder belt side seat belt sliding surface extends from a shoulder belt side of a position at which a distance in a direction perpendicular or substantially perpendicular to the length direction of the seat belt insertion hole is minimized to a lap belt side of the position, and a portion of the shoulder belt side seat belt sliding surface is located on the lap belt side of the position at which the distance is minimized.