In a tongue used in a seat belt system, a tongue main body 25 including a belt insertion hole 23 in which to insert a seat belt 3 is provided with an insertion part 27 in a projecting manner. The insertion part 27 is capable of being inserted in a buckle in the seat belt system, and includes a lock hole 27a to which a lock piece included in the buckle is capable of being locked. A front surface of the insertion part 27 is formed into a concave and convex surface by being provided with multiple convex parts 29 or multiple concave parts. The convex parts 29 or the concave parts included in a rear surface of the tongue are formed corresponding to the convex parts 29 or the concave parts included in the front surface. A height of each convex part 29 from a reference position or a depth of each concave part from the reference position is 0.01 mm to 0.10 mm.