A side airbag device stowed in a seatback of a vehicle seat, comprising: an inflator arranged on the occupant side of a side frame positioned on a side part of the seatback, which generates expansion gas; and an airbag that expands due to said expansion gas and protects the occupant seated in the vehicle seat, wherein the airbag includes a main chamber that deploys forward of the seatback, and a pre-chamber stowing the inflator in an inner portion thereof and which is connected to the occupant side of the main chamber, and an integral joint with no gaps present is formed between at least a front edge part of the pre-chamber and the main chamber, wherein the main chamber includes an inner main panel and an outer main panel, is formed by overlapping said two panels (32a,32b) and sewing the outer edges thereof, and where the pre-chamber is formed by sewing one pre-chamber panel to the inner main panel, wherein a first inner vent is formed on the inner main panel so that expansion gas can flow from the pre-chamber to the main chamber via said first inner vent, wherein a baffle panel having an inner edge part connected to the inner surface of the inner main panel and an outer edge part connected to the inner surface of the outer main panel is provided, and the baffle panel separates the main chamber into a forward main chamber and a rear main chamber, wherein a second inner vent is formed on the baffle panel so that expansion gas can flow from the rear main chamber to the forward main chamber via said second inner vent, characterized in that - the integral joint is made of four overlapping layers, being the inner main panel, the front edge part of the pre-chamber panel, and the inner edge part of the baffle panel, and a reinforcement cloth provided between the front edge part of the pre-chamber panel and the outer surface of the inner main panel. an inflator arranged on the occupant side of a side frame positioned on a side part of the seatback, which generates expansion gas; and an airbag that expands due to said expansion gas and protects the occupant seated in the vehicle seat, wherein the airbag includes a main chamber that deploys forward of the seatback, and a pre-chamber stowing the inflator in an inner portion thereof and which is connected to the occupant side of the main chamber, and an integral joint with no gaps present is formed between at least a front edge part of the pre-chamber and the main chamber, wherein the main chamber includes an inner main panel and an outer main panel, is formed by overlapping said two panels (32a,32b) and sewing the outer edges thereof, and where the pre-chamber is formed by sewing one pre-chamber panel to the inner main panel, wherein a first inner vent is formed on the inner main panel so that expansion gas can flow from the pre-chamber to the main chamber via said first inner vent, wherein a baffle panel having an inner edge part connected to the inner surface of the inner main panel and an outer edge part connected to the inner surface of the outer main panel is provided, and the baffle panel separates the main chamber into a forward main chamber and a rear main chamber, wherein a second inner vent is formed on the baffle panel so that expansion gas can flow from the rear main chamber to the forward main chamber via said second inner vent, characterized in that - the integral joint is made of four overlapping layers, being the inner main panel, the front edge part of the pre-chamber panel, and the inner edge part of the baffle panel, and a reinforcement cloth provided between the front edge part of the pre-chamber panel and the outer surface of the inner main panel. - the integral joint is made of four overlapping layers, being the inner main panel, the front edge part of the pre-chamber panel, and the inner edge part of the baffle panel, and a reinforcement cloth provided between the front edge part of the pre-chamber panel and the outer surface of the inner main panel.