To ensure low-cost manufacture in a simple structure and prevent release of a tong with slight force in the direction of releasing the tong.

A seat belt buckle used with a tong 16 contains a frame 10 and an ejector 20 is provided in the frame 10 to be operated by an ejector spring 22. With the insertion of the tong 16, a retainer 30 is pushed down along a track 26 against the operation of the retainer spring 32. The track 26 is inclined at an angle α to a passage for the tong 16 and extended in the direction A of inserting the tong 16 to be apart from the passage to the outside. When the tong 16 is inserted in proper amount, the retainer 30 is moved up along a slope 28 with the spring 32 and engaged with the opening 18 of the tong 16 to hold the tong 16 in the locked condition. The retainer 30 is released from the opening 8 with the operation of a releasing mechanism to release the tong 16.