A side airbag device comprising: a side airbag that is provided at a side portion of a seatback, and includes a rear inflation section that inflates and deploys at the side of at least rear portions of the chest and abdominal regions of a seated occupant on receipt of supplied gas, and by a front inflation section that inflates and deploys at the side of at least front portions of the chest and abdominal regions of the seated occupant on receipt of gas supplied from the rear inflation section through a communication path, and characterized in that said communication path is formed at a boundary section that segments the side airbag into the rear inflation section and the front inflation section; and a tension member (24, 46, 92, 104) that receives tension and deploys accompanying inflation and deployment of the side airbag, wherein in a deployed state, a front end side of the tension member (24, 46, 92, 104) in the vehicle front-rear direction is connected to the rear inflation section, and the rear end side of the tension member (24, 46, 92, 104) in the vehicle front-rear direction is connected to a frame of the seatback, and in that the rear inflation section, in an inflated and deployed state, includes a shoulder restraining section that projects out further forward than an inflated and deployed portion positioned at the side of the rear portions of the chest and abdominal regions of the seated occupant to a position at the side of the shoulder region of the seated occupant; and in the inflated and deployed state of the rear inflation section, a front end side of the tension member (24, 92, 104) is connected to a front end portion of the shoulder restraining section in the vehicle front-rear direction. a side airbag that is provided at a side portion of a seatback, and includes a rear inflation section that inflates and deploys at the side of at least rear portions of the chest and abdominal regions of a seated occupant on receipt of supplied gas, and by a front inflation section that inflates and deploys at the side of at least front portions of the chest and abdominal regions of the seated occupant on receipt of gas supplied from the rear inflation section through a communication path, and characterized in that said communication path is formed at a boundary section that segments the side airbag into the rear inflation section and the front inflation section; and a tension member (24, 46, 92, 104) that receives tension and deploys accompanying inflation and deployment of the side airbag, wherein in a deployed state, a front end side of the tension member (24, 46, 92, 104) in the vehicle front-rear direction is connected to the rear inflation section, and the rear end side of the tension member (24, 46, 92, 104) in the vehicle front-rear direction is connected to a frame of the seatback, and in that the rear inflation section, in an inflated and deployed state, includes a shoulder restraining section that projects out further forward than an inflated and deployed portion positioned at the side of the rear portions of the chest and abdominal regions of the seated occupant to a position at the side of the shoulder region of the seated occupant; and in the inflated and deployed state of the rear inflation section, a front end side of the tension member (24, 92, 104) is connected to a front end portion of the shoulder restraining section in the vehicle front-rear direction.