A side airbag device that is stowed inside a vehicle seat and protects an occupant seated in the seat, including an airbag that restrains the occupant by expanding and deploying, and an inflator that supplies expansion gas to the airbag. The airbag includes a pre-push chamber that contains the inflator inside and deploys mainly on the occupant side (inside), and a main chamber that is connected to the pre-push chamber and deploys mainly toward the front of the vehicle. When the airbag is in the stowed state, the pre-push chamber forms the first folded part and the main chamber forms the second folded part. At the portion where the first folded part and the second folded part overlap, a sewn part joins the first folded part and at least a portion of the pre-chamber, and the sewn part is configured to break during deployment of the airbag.