To provide a seatbelt system with its tongue plate locked fast even when an inertia force in the operating direction of the unlocking member is imposed on the buckle.

The buckle main body 11 for removably locking the tongue plate 7 is equipped with an auxiliary lock member 19 supported by a latch member 17, the latter supported by a base member 14 and rotatable between the lock position and unlock position, and capable of holding the latch member 17 at the lock position; a control member 20 movably supported by the latch member 17 for controlling the movement of the auxiliary lock member 19; an unlock button 21 for releasing the locked state; and a protect member 29 movably held by a spring holder 23 attached to the base member 14 and energized in the direction opposite to the unlock direction. The control member 20 in its 1st behavior rotates in response to the rotation of the latch member 17, and in its 2nd behavior moves roughly in the direction of the extraction of the tongue plate 7. The protect member 29, when exposed to the inertia force of the releasing direction, resists the energization and moves, this movement impeding the 1st behavior.