A side air bag device (10) includes an inflator (11) arranged in a
backrest (15a) of a seat (15) to discharge gas, an air bag (12) having an
upper chamber (12a) and a lower chamber (12b), which are each inflated by
discharging gas, and an impact reducing means (19) for reducing impact
that is applied to a passenger when the air bag contacts the passenger
during inflation of the air bag.