A vehicular seat installed with a side airbag device, comprising: a seat back including a side frame and a side portion on an outer side in a vehicle width direction; an inflator placed on the side frame in the vehicle width direction, within the side portion of the seat back; an outside bag placed on the outer side of the side frame in the vehicle width direction, within the side portion, the outside bag being adapted to inflate and deploy between a side portion of a vehicle interior and an occupant, when gas is supplied from the inflator housed in the outside bag; an inside bag connected to an inner portion of the outside bag in the vehicle width direction, on a vehicle front side of the side frame, the inside bag being adapted to inflate and deploy in the side portion when gas is supplied from the inflator, the inside bag being adapted to complete inflation and deployment earlier than the outside bag, so as to press the occupant inward in the vehicle width direction; characterized in that: the vehicular seat further comprises: a reaction force plate that extends in a vehicle front-back direction as viewed in a plane section of the seat back, the reaction force plate being supported by the side frame such that the reaction force plate extends toward a front of the vehicle beyond a front edge portion of the side frame as viewed in a side view of the vehicle, the reaction force plate being adapted to apply reaction force inward in the vehicle width direction, to the inside bag, at a time of inflation and deployment; wherein: the inflator is placed on an outer side of the frame; and the inside bag is adapted to inflate and deploy at an upper level than an abdominal region of the occupant, so that the abdominal region, having a relatively low load tolerance, is not pressed by the inside bag. a seat back including a side frame and a side portion on an outer side in a vehicle width direction; an inflator placed on the side frame in the vehicle width direction, within the side portion of the seat back; an outside bag placed on the outer side of the side frame in the vehicle width direction, within the side portion, the outside bag being adapted to inflate and deploy between a side portion of a vehicle interior and an occupant, when gas is supplied from the inflator housed in the outside bag; an inside bag connected to an inner portion of the outside bag in the vehicle width direction, on a vehicle front side of the side frame, the inside bag being adapted to inflate and deploy in the side portion when gas is supplied from the inflator, the inside bag being adapted to complete inflation and deployment earlier than the outside bag, so as to press the occupant inward in the vehicle width direction; characterized in that: the vehicular seat further comprises: a reaction force plate that extends in a vehicle front-back direction as viewed in a plane section of the seat back, the reaction force plate being supported by the side frame such that the reaction force plate extends toward a front of the vehicle beyond a front edge portion of the side frame as viewed in a side view of the vehicle, the reaction force plate being adapted to apply reaction force inward in the vehicle width direction, to the inside bag, at a time of inflation and deployment; wherein: the inflator is placed on an outer side of the frame; and the inside bag is adapted to inflate and deploy at an upper level than an abdominal region of the occupant, so that the abdominal region, having a relatively low load tolerance, is not pressed by the inside bag. the inflator is placed on an outer side of the frame; and the inside bag is adapted to inflate and deploy at an upper level than an abdominal region of the occupant, so that the abdominal region, having a relatively low load tolerance, is not pressed by the inside bag.