SIDE AIRBAG DEVICE

A side airbag device includes: a main bag is built in a seat of a vehicle, and inflates and deploys toward a side of an occupant and a vehicle front side; an inner bag which is formed in a bag shape independent of the main bag and inflates and deploys on a vehicle rear side inside the main bag An inflator provided inside the inner bag to supply gas; and a tension member which spans between the vehicle front side and the vehicle rear side inside the inner bag and is tensioned as the inner bag inflates and deploys. The length of the inner bag in the front-back direction of the vehicle at inflation is restricted by the tension member to a predetermined range, and the inner bag therefore inflates in a vehicle width direction and pushes and spreads the vehicle rear side of the main bag from the inside of the main bag to both sides in the vehicle width direction.