A side airbag device having a plurality of chambers. The side airbag 100 includes: an inflator 110; a rear chamber 114 which deploys at a side part 108 of a seat back 106 and a middle restraining part 122 that protrudes toward the front of a vehicle and a main part 120 at a rear side. A sectioning wall 124 which constitutes a front edge and an upper edge of the rear chamber 114. A front chamber 116 inflates into areas toward the front of the vehicle and the upper part of the vehicle from the sectioning wall 124. An upper opening 126 is formed on the sectioning wall 124 in a rear part of the upper edge of the rear chamber 114. A lower opening 128 is formed on the sectioning wall 124 in a lower part of the front edge of the rear chamber 114.