PURPOSE: To prevent the improper buckling of seat belts of adjacent seat belt devices by forming a difference in shape for one of the adjacent buckle devices in a relationship between the tongue insertion hole of a buckle cover and a tongue.

CONSTITUTION: In the case of a plurality of seat belts arranged in parallel, the buckle device of one of the seat belts is so shaped as to have curved parts 16 ana 16' projected in a thickness-wise direction at least in one part of the width-wise direction thereof without any change in width for the other buckle device. A buckle side opening 1d for receiving the curved parts 16 and 16' is made to match a tongue insertion shape 11a, and an opening 11 is formed on a buckle cover 12. The curved parts 16 and 16' formed on a tongue 13 are made to have a sectional form of insertion part different from the flat tongue 1 of the other buckle device.