A vehicle seat provided with a side airbag device, the side air bag device comprising: an inflator fixed to a side frame extending in a vehicle vertical direction inside a side portion of a seatback of the vehicle seat; and an airbag (5; 50; 500) configured to be deployed by inflation gas supplied from the inflator, wherein the airbag (5; 50; 500) comprises an inside base cloth which is provided to face an occupant side, an outside base cloth which is provided to face an outward side, in a vehicle width direction, of the inside base cloth and to be joined to the inside base cloth at a whole peripheral portion thereof, and a regulating member (8; 80) which is provided between the inside base cloth and the outside base cloth, and the airbag (5; 50; 500) has a side-portion protection area which is configured to be deployed at the occupant side in the vehicle vertical direction and a back-portion protection area which is configured to be deployed between a back portion of an occupant and the seatback, the regulating member (8; 80) is respectively joined to a front portion and a rear portion of the inside base cloth at a front-side joint portion and a rear-side joint portion, the length, in a vehicle longitudinal direction, between the front-side joint portion and the rear-side joint portion of the regulating member (8; 80) is set to be shorter than that between the front-side joint portion and the rear-side joint portion of the inside base cloth, whereby a joint state of the inside base cloth is loosened relative to the regulating member (8; 80), the rear-side joint portion is configured, in an airbag deployment state, to be positioned on a vehicle forward side of a front end portion of the side frame, and the back-portion protection area of the airbag (5; 50; 500) is constituted by a part of the inside base cloth, forward deployment of which is regulated by the regulating member (8; 80), characterized in that the airbag (5; 50; 500) is configured such that the back-portion protection area is positioned on an inward side, in the vehicle width direction, of the side frame and the side-portion protection area is positioned on the outward side, in the vehicle width direction, of the side frame in a state where the airbag is stored inside the side portion of the seatback before deployment. an inflator fixed to a side frame extending in a vehicle vertical direction inside a side portion of a seatback of the vehicle seat; and an airbag (5; 50; 500) configured to be deployed by inflation gas supplied from the inflator, wherein the airbag (5; 50; 500) comprises an inside base cloth which is provided to face an occupant side, an outside base cloth which is provided to face an outward side, in a vehicle width direction, of the inside base cloth and to be joined to the inside base cloth at a whole peripheral portion thereof, and a regulating member (8; 80) which is provided between the inside base cloth and the outside base cloth, and the airbag (5; 50; 500) has a side-portion protection area which is configured to be deployed at the occupant side in the vehicle vertical direction and a back-portion protection area which is configured to be deployed between a back portion of an occupant and the seatback, the regulating member (8; 80) is respectively joined to a front portion and a rear portion of the inside base cloth at a front-side joint portion and a rear-side joint portion, the length, in a vehicle longitudinal direction, between the front-side joint portion and the rear-side joint portion of the regulating member (8; 80) is set to be shorter than that between the front-side joint portion and the rear-side joint portion of the inside base cloth, whereby a joint state of the inside base cloth is loosened relative to the regulating member (8; 80), the rear-side joint portion is configured, in an airbag deployment state, to be positioned on a vehicle forward side of a front end portion of the side frame, and the back-portion protection area of the airbag (5; 50; 500) is constituted by a part of the inside base cloth, forward deployment of which is regulated by the regulating member (8; 80), characterized in that the airbag (5; 50; 500) is configured such that the back-portion protection area is positioned on an inward side, in the vehicle width direction, of the side frame and the side-portion protection area is positioned on the outward side, in the vehicle width direction, of the side frame in a state where the airbag is stored inside the side portion of the seatback before deployment.