To provide a side airbag device for a rear seat capable of performing adequate impact suppression to an occupant seated in the rear seat having a reclining mechanism in side collision.

A backrest part 12a of a rear seat 12 is constituted in a tiltable arranging manner to the position where a part thereof overlaps a body side part other than a side door when viewed from the side of a vehicle and to the 100% overlapping position where the entire backrest overlaps the body side part other than the side door. An inflator 18 has two chambers. In a side impact, when the detection signal that the backrest part 12a is arranged at the 100% overlapping position is output from a detection means 27, the inflator 18 is operated by the command signal from a control unit 25 so that gas is generated only from one chamber, and the internal pressure in an airbag 17 in a deployed state is dropped.