A side airbag device includes: an airbag which is stored in a storage part provided on a side of an occupant sitting on a seat of a vehicle in a normal posture, and is deployed and inflated forward between the occupant and a side wall part by an inflation gas supplied from a gas generator in response to an impact applied from a side to the side wall part of the vehicle; an exhaust port configured to exhaust an inflation gas, the exhaust port being provided with the airbag and open toward a direction along a wall surface of the side wall part; and an opening varying mechanism configured to reduce an opening area of the exhaust port in a later stage of deployment and inflation of the airbag compared to an initial stage.