SIDE AIR-BAG DEVICE

Provided is a side airbag device which is capable of further stabilizing a deployment behavior of a cushion having a plurality of chambers. A side airbag 100 includes: an inflator 110; a rear chamber 114 which inflates and deploys at a side part 108 of a seat back 106 and which includes a middle restraining part 122 that protrudes toward the front of a vehicle from the seat back 106 to restrain a part of an occupant 118 and a main part 120 positioned at the rear side of the vehicle than the middle restraining part 122; a sectioning wall 124 which constitutes a front edge and an upper edge of the rear chamber 114 and which has a prescribed width in a vehicle width direction; a front chamber 116 which inflates and deploys into areas toward the front of the vehicle and the upper part of the vehicle from the sectioning wall 124; an upper opening 126 which is formed on the sectioning wall 124 in a rear part of the upper edge of the rear chamber 114; and a lower opening 128 which is formed on the sectioning wall 124 in a lower part of the front edge of the rear chamber 114.