an inflatable seat belt apparatus is formed of an inflatable portion which is usually maintained in a band shape and has an envelope part inflated and deployed in a spindle shape by introducing gas generated by a gas generating device , the gas generating device acting in case of exceeding a predetermined threshold , a webbing having the inflatable portion extending over at least a range directly contacting with an occupant , a tongue fixed to an end of the inflatable portion and having a gas path from the gas generating device and a buckle to which the tongue is detachably engaged . the inflatable portion has an elastic inflatable unit inserted into the envelope part and is inflated and deployed by introducing the gas generated by the gas generating device into the elastic inflatable unit via a filter .