A side airbag device that is disposed inside a side of a backrest part of a seat and that is deployed an area between a side door and the seat or an area between adjacent seats, the side airbag device comprising: an airbag; and an inflator that is disposed at an end of a retreat direction side of a vehicle in the airbag when the airbag is deployed and that generates a gas by receiving a signal output from a sensor when a side collision or an overturn occurs and supplies the gas to the airbag in a folded state so as to deploy the airbag, wherein the airbag is formed in a bag shape by connecting a periphery of a main base cloth of a vehicle inner side and a periphery of a main base cloth of a vehicle outer side, in an inside of the airbag formed in the bag shape, the airbag includes: a first partition wall partitions the inside into at least a front chamber located at an advance direction side of the vehicle and a rear chamber located at the retreat direction side of the vehicle at a time of the deployment of the airbag; and a first inner tether (5a, 5b) connecting between an end of the advance direction side of the vehicle of the front chamber and the first partition wall in the front chamber, characterized in that a length of the first inner tether (5a, 5b) is shorter than a length in an advance and retreat direction of the vehicle between a first joint part connecting the first partition wall to the first inner tether (5a, 5b) and the end of the advance direction side of the vehicle of the front chamber in the airbag having a state prior to the deployment of the airbag,. wherein an intermediate portion of the first partition wall in a vehicle width direction is provided to protrude in the advance direction of the vehicle at a time of the deployment of the airbag, and an intermediate portion in a vehicle height direction of the first partition wall in the protruded state is the first joint part. an airbag; and an inflator that is disposed at an end of a retreat direction side of a vehicle in the airbag when the airbag is deployed and that generates a gas by receiving a signal output from a sensor when a side collision or an overturn occurs and supplies the gas to the airbag in a folded state so as to deploy the airbag, wherein the airbag is formed in a bag shape by connecting a periphery of a main base cloth of a vehicle inner side and a periphery of a main base cloth of a vehicle outer side, in an inside of the airbag formed in the bag shape, the airbag includes: a first partition wall partitions the inside into at least a front chamber located at an advance direction side of the vehicle and a rear chamber located at the retreat direction side of the vehicle at a time of the deployment of the airbag; and a first inner tether (5a, 5b) connecting between an end of the advance direction side of the vehicle of the front chamber and the first partition wall in the front chamber, characterized in that a first partition wall partitions the inside into at least a front chamber located at an advance direction side of the vehicle and a rear chamber located at the retreat direction side of the vehicle at a time of the deployment of the airbag; and a first inner tether (5a, 5b) connecting between an end of the advance direction side of the vehicle of the front chamber and the first partition wall in the front chamber, characterized in that a length of the first inner tether (5a, 5b) is shorter than a length in an advance and retreat direction of the vehicle between a first joint part connecting the first partition wall to the first inner tether (5a, 5b) and the end of the advance direction side of the vehicle of the front chamber in the airbag having a state prior to the deployment of the airbag,. wherein an intermediate portion of the first partition wall in a vehicle width direction is provided to protrude in the advance direction of the vehicle at a time of the deployment of the airbag, and an intermediate portion in a vehicle height direction of the first partition wall in the protruded state is the first joint part.