A buckle for a seat belt device has a latch member that is configured to be engaged with a tongue when the tongue is inserted into the buckle; a buckle base having a bottom wall and a first side wall and a second side wall that stand up from the opposite side edges of the bottom wall, each of the first and second side walls having latch holding portions that hold the latch member, the latch holding portions each being formed of a cutout extending from an upper end of each of the first and second side walls of the buckle base; and a bridging member configured to be assemblable to the first and second side walls so as to span the latch holding portions in a front-rear direction.