A tongue for a seat belt system. The tongue is configured to allow a seat belt to smoothly slide thereon at normal times and that is capable of preventing stretch of a lap belt in the event of emergency. The seat belt sliding unit includes a predetermined number of the protrusions disposed in the first and second concave portions and. The first protrusion has a flat or substantially flat end surface on the shoulder belt side, a flat or substantially flat end surface on the lap belt side, and an arc-like top end. An angle formed by the imaginary line between the topmost end of the protrusion and an arc center of an outer peripheral surface of the concave portions and the end surface on the shoulder belt side is less than an angle formed by the imaginary line and the end surface on the lap belt side.