A side airbag device which restrains an occupant of a vehicle seat from sideways, the side airbag device comprising: an inflator which generates gas; and a cushion which receives the gas and inflates and deploys at the side of a seat back of the vehicle seat during a vehicle emergency, wherein the cushion includes: a rear chamber having a middle restraining part, at least a part of which protrudes from the seat back toward the front of the vehicle to restrain a part of a body of the occupant, and a main part positioned at a rear side of the vehicle than the middle restraining part; a sectioning wall which constitutes a front edge and an upper edge of the rear chamber and which has a prescribed width in a vehicle width direction; a front chamber which inflates and deploys into areas toward the front of the vehicle and the upper part of the vehicle from the sectioning wall; an upper opening which is formed on the sectioning wall in a rear part of the upper edge of the rear chamber and which is capable of supplying gas from the rear chamber to the front chamber; and a lower opening which is formed on the sectioning wall in a lower part of the front edge of the rear chamber and which is capable of supplying gas from the rear chamber to the front chamber, characterized in that the rear chamber further includes: a partition which divides the main part and the middle restraining part from each other, which is provided so as to be positioned along a vehicle front edge of a side part of the seat back when the cushion inflates and deploys, and which has a prescribed width in the vehicle width direction; and a prescribed partition vent hole which is provided on the partition. an inflator which generates gas; and a cushion which receives the gas and inflates and deploys at the side of a seat back of the vehicle seat during a vehicle emergency, wherein the cushion includes: a rear chamber having a middle restraining part, at least a part of which protrudes from the seat back toward the front of the vehicle to restrain a part of a body of the occupant, and a main part positioned at a rear side of the vehicle than the middle restraining part; a sectioning wall which constitutes a front edge and an upper edge of the rear chamber and which has a prescribed width in a vehicle width direction; a front chamber which inflates and deploys into areas toward the front of the vehicle and the upper part of the vehicle from the sectioning wall; an upper opening which is formed on the sectioning wall in a rear part of the upper edge of the rear chamber and which is capable of supplying gas from the rear chamber to the front chamber; and a lower opening which is formed on the sectioning wall in a lower part of the front edge of the rear chamber and which is capable of supplying gas from the rear chamber to the front chamber, characterized in that the rear chamber further includes: a partition which divides the main part and the middle restraining part from each other, which is provided so as to be positioned along a vehicle front edge of a side part of the seat back when the cushion inflates and deploys, and which has a prescribed width in the vehicle width direction; and a prescribed partition vent hole which is provided on the partition. a rear chamber having a middle restraining part, at least a part of which protrudes from the seat back toward the front of the vehicle to restrain a part of a body of the occupant, and a main part positioned at a rear side of the vehicle than the middle restraining part; a sectioning wall which constitutes a front edge and an upper edge of the rear chamber and which has a prescribed width in a vehicle width direction; a front chamber which inflates and deploys into areas toward the front of the vehicle and the upper part of the vehicle from the sectioning wall; an upper opening which is formed on the sectioning wall in a rear part of the upper edge of the rear chamber and which is capable of supplying gas from the rear chamber to the front chamber; and a lower opening which is formed on the sectioning wall in a lower part of the front edge of the rear chamber and which is capable of supplying gas from the rear chamber to the front chamber, a partition which divides the main part and the middle restraining part from each other, which is provided so as to be positioned along a vehicle front edge of a side part of the seat back when the cushion inflates and deploys, and which has a prescribed width in the vehicle width direction; and a prescribed partition vent hole which is provided on the partition.