A side airbag device comprising: a vertically long rear chamber that inflates and deploys from a side portion of a vehicle seat; a front chamber that abuts a front edge portion of the rear chamber and inflates and deploys forward and upward from the rear chamber; a vertically long inflator contained within a rear part of the rear chamber; and a cloth-like first tether that is provided outside of the rear chamber, has a front end connected to the front edge portion of the rear chamber and a rear end connected to the rear part of the rear chamber, and is tensioned when the rear chamber is inflated and deployed; wherein: the rear chamber has a smaller capacity than the front chamber; wherein the first tether is longer in a vertical direction than in a front-back direction, and the front end of the first tether is connected to a range that includes at least a lower end of the front edge portion of the rear chamber; the side airbag device further comprising two stud bolts (110a,110b) provided longitudinally spaced apart at the inflator, exposed from the rear chamber, and configured to be fastened to the vehicle seat wherein the rear end of the first tether is anchored to the rear part of the rear chamber across the two stud bolts (110a, 110b), and the front end of the first tether has a length greater than or equal to a distance between the two stud bolts (110a, 110b), and the front end of the first tether is connected to a range of the front edge portion of the rear chamber, the range including a location of the shortest distance to the stud bolts (110a, 110b); characterized in that a cloth-like second tether that is provided outside of the front chamber and the rear chamber, has a front end connected to a front edge portion of the front chamber and a rear end connected to a rear part of the rear chamber, and is tensioned when the rear chamber is inflated and deployed, wherein the rear end of the second tether is anchored by at least one of the two stud bolts (110a, 110b) at the rear part of the rear chamber wherein the second tether is longer in the front-back direction than in the vertical direction. a vertically long rear chamber that inflates and deploys from a side portion of a vehicle seat; a front chamber that abuts a front edge portion of the rear chamber and inflates and deploys forward and upward from the rear chamber; a vertically long inflator contained within a rear part of the rear chamber; and a cloth-like first tether that is provided outside of the rear chamber, has a front end connected to the front edge portion of the rear chamber and a rear end connected to the rear part of the rear chamber, and is tensioned when the rear chamber is inflated and deployed; wherein: the rear chamber has a smaller capacity than the front chamber; wherein the first tether is longer in a vertical direction than in a front-back direction, and the front end of the first tether is connected to a range that includes at least a lower end of the front edge portion of the rear chamber; the side airbag device further comprising two stud bolts (110a,110b) provided longitudinally spaced apart at the inflator, exposed from the rear chamber, and configured to be fastened to the vehicle seat wherein the rear end of the first tether is anchored to the rear part of the rear chamber across the two stud bolts (110a, 110b), and the rear chamber has a smaller capacity than the front chamber; wherein the first tether is longer in a vertical direction than in a front-back direction, and the front end of the first tether is connected to a range that includes at least a lower end of the front edge portion of the rear chamber; the side airbag device further comprising two stud bolts (110a,110b) provided longitudinally spaced apart at the inflator, exposed from the rear chamber, and configured to be fastened to the vehicle seat wherein the rear end of the first tether is anchored to the rear part of the rear chamber across the two stud bolts (110a, 110b), and the front end of the first tether has a length greater than or equal to a distance between the two stud bolts (110a, 110b), and the front end of the first tether is connected to a range of the front edge portion of the rear chamber, the range including a location of the shortest distance to the stud bolts (110a, 110b); characterized in that a cloth-like second tether that is provided outside of the front chamber and the rear chamber, has a front end connected to a front edge portion of the front chamber and a rear end connected to a rear part of the rear chamber, and is tensioned when the rear chamber is inflated and deployed, wherein the rear end of the second tether is anchored by at least one of the two stud bolts (110a, 110b) at the rear part of the rear chamber wherein the second tether is longer in the front-back direction than in the vertical direction.