A gas generator is arranged in a vacant space around the seat to achieve effective utilization of a space for the seat. An inflatable seat bag is arranged above a seat pan. A seat cushion is installed above the seat bag. A gas introduction port is provided at a longitudinal end (the side of a side frame) of the seat bag, and a gas supply duct from a manifold at a distal end of an inflator is connected to the gas introduction port. The gas supply duct is disposed along the lower side of an S-spring. The inflator is mounted to a spindle with a bracket. A distal end of a gas introduction duct is connected to an end portion of a lap bag on the side of an anchor. A proximal end of the duct is connected to another gas outlet of the manifold.