Ducts 66 are housed in a housing 46 of a buckle 12. When plugs 20 of a tongue 10 are inserted into plug receiving bores 48, gas outlets 72 of ducts 66 engage plugs 20. Check doors 50 that open and close are arranged on the plug receiving bores 48 of the buckle 12. Check covers 74 including flexible pieces 76 are arranged at tips of the ducts 66 and are open by gas pressure. When a gas generator 16 is actuated with the tongue 10 separated from the buckle 12, gas is discharged downward from gas relief ports 60. The check doors 50 and the check covers 74 prevent the entrance of foreign matters.