A seat belt device includes a buckle provided on a side portion of a seat for a vehicle, and configured such that a tongue plate is inserted into the buckle; a buckle force limiter mechanism connected to the buckle via a connecting member having an elongate shape, the buckle force limiter mechanism being configured to allow the buckle to move in a pulling direction when a tensile load equal to or larger than a predetermined tensile load is applied to the buckle; and a case covering the connecting member from an inner side in a seat width direction, the case having a corner at an upper end portion of the case, the corner facing the connecting member, and the corner having an arc shape.