A seat with a side airbag device comprising: an airbag cushion which is built in the seat of a vehicle and inflates and deploys using gas toward a side of an occupant sitting in the seat and a vehicle front side; an inflator having a cylindrical shape and having one end side inserted into the airbag cushion to supply gas to the airbag cushion; and the inflator has: a gas ejection hole which is disposed inside the airbag cushion to eject gas; a connector which is provided on the other end side and to which a predetermined harness is connected; and a stud bolt which is provided at a predetermined location and is fastened to the seat, the airbag cushion has: a chamber formed in a bag shape; an extension portion which extends from the chamber on a vehicle rear side and into which the one end side of the inflator is inserted; and a predetermined tab (118, 172, 192) which extends from the chamber in the vicinity of the extension portion and is held in the vicinity of the other end side of the inflator, characterized in that a predetermined structure which is built in the seat, extends in a vehicle width direction and is fixed to the seat, wherein the predetermined structure is disposed so as to pass through a space portion surrounded by the chamber, the extension portion, the inflator, and the tab (118, 172, 192), and the airbag cushion is attached to the seat by the stud bolt. an airbag cushion which is built in the seat of a vehicle and inflates and deploys using gas toward a side of an occupant sitting in the seat and a vehicle front side; an inflator having a cylindrical shape and having one end side inserted into the airbag cushion to supply gas to the airbag cushion; and the inflator has: a gas ejection hole which is disposed inside the airbag cushion to eject gas; a connector which is provided on the other end side and to which a predetermined harness is connected; and a stud bolt which is provided at a predetermined location and is fastened to the seat, the airbag cushion has: a chamber formed in a bag shape; an extension portion which extends from the chamber on a vehicle rear side and into which the one end side of the inflator is inserted; and a predetermined tab (118, 172, 192) which extends from the chamber in the vicinity of the extension portion and is held in the vicinity of the other end side of the inflator, characterized in that a predetermined structure which is built in the seat, extends in a vehicle width direction and is fixed to the seat, wherein the predetermined structure is disposed so as to pass through a space portion surrounded by the chamber, the extension portion, the inflator, and the tab (118, 172, 192), and the airbag cushion is attached to the seat by the stud bolt. a gas ejection hole which is disposed inside the airbag cushion to eject gas; a connector which is provided on the other end side and to which a predetermined harness is connected; and a stud bolt which is provided at a predetermined location and is fastened to the seat, the airbag cushion has: a chamber formed in a bag shape; an extension portion which extends from the chamber on a vehicle rear side and into which the one end side of the inflator is inserted; and a predetermined tab (118, 172, 192) which extends from the chamber in the vicinity of the extension portion and is held in the vicinity of the other end side of the inflator, characterized in that a predetermined structure which is built in the seat, extends in a vehicle width direction and is fixed to the seat, wherein the predetermined structure is disposed so as to pass through a space portion surrounded by the chamber, the extension portion, the inflator, and the tab (118, 172, 192), and the airbag cushion is attached to the seat by the stud bolt. a chamber formed in a bag shape; an extension portion which extends from the chamber on a vehicle rear side and into which the one end side of the inflator is inserted; and a predetermined tab (118, 172, 192) which extends from the chamber in the vicinity of the extension portion and is held in the vicinity of the other end side of the inflator, characterized in that a predetermined structure which is built in the seat, extends in a vehicle width direction and is fixed to the seat, wherein the predetermined structure is disposed so as to pass through a space portion surrounded by the chamber, the extension portion, the inflator, and the tab (118, 172, 192), and the airbag cushion is attached to the seat by the stud bolt.