A side airbag device including an airbag (2); an inflator (3); and an inner tube (4) formed of a fabric and covering discharge holes (3a) of the inflator (3). The inner tube (4) protrudes toward a center of the airbag (2) in a deployed state and a first end (4b) is closed in the direction of gas discharge, and second ends (4c) that are in a vehicle height direction of the airbag (2) in the deployed state are open. The inner tube (4) is folded back at a folded-back portion (4d) inside the airbag (2) when the airbag (2) is folded in a case where the side airbag device is installed in a vehicle seat back.