To provide a seat belt buckle mounting structure which provides a reasonable constitution by reducing the number of components for attaching a buckle of a seat belt device to a seat cushion.

A buckle 5 forming a receiving piece of a seat belt device is fitted in a seat cushion. In the seat cushion, a side part on the inner side with the buckle 5 being arranged thereon is supported by a vehicle body floor by a supporting frame 50 which is formed of a plate in a bent manner. The supporting frame 50 comprises: a right surface part 52 fixed to the seat cushion; a lower surface part 51 fixed to an upper rail 10 (a vehicle body floor) of a slide mechanism 4; and a left surface part 53 with the buckle 5 being fitted thereto. The lower surface part 51 is folded back horizontally from a first fold-back part 50A hanging from the right surface part 52. The left surface part 53 is folded back directly upward of a second fold-back part 50B extending from the lower surface part 51 to the inner side with the buckle 5 being arranged thereon.