The present invention is a side airbag device that is housed in the side support part of a vehicle seat and includes an airbag for restraining an occupant and an inflator. The airbag includes a first bag housing the inflator which is capable of inflating and deploying, and a second bag capable of inflating and deploying on the side of the first bag. The second bag is configured to be deployed by inflation gas flowing in from the first bag. At the upper edge of the second bag, an inclined part is formed that is inclined to be lower at the front thereof than at the rear thereof when viewed from the vehicle width direction. The front portion of the inclined part is configured so as to be located lower than the center line extending in the longitudinal direction of the upper arm of the occupant seated in a vehicle seat.