To provide a side airbag device configured so that an airbag can be expanded to a desired position, with a simple structure.SOLUTION: The side airbag device comprises an airbag which has an inflator, a first inflation part that inflates and expands beside an occupant and is fixed to a seat when inflating and expanding and a second inflation part inclining toward the occupant with respect to the first inflation part. The airbag has an occupant-side panel arranged at the occupant side, an occupant opposite-side panel arranged to oppose the occupant-side panel, and a belt-like connection panel connecting an outer peripheral edge of the occupant-side panel to an outer peripheral edge of the occupant opposite-side panel. The occupant opposite-side panel is arranged with an interval in a vehicle width direction with respect to the occupant-side panel. The connection panel has a bent part, between a first outer peripheral edge part connected to the outer peripheral edge of the occupant-side panel forming the first inflation part and a second outer peripheral edge part connected to the outer peripheral edge of the occupant-side panel forming the second inflation part or at the second outer peripheral edge part.SELECTED DRAWING: Figure 4