To provide a side airbag device capable of controlling the inner pressure of an upper bag for protecting a chest section formed on the top of a side airbag so as to be partitioned, to its proper pressure, corresponding to a damaged condition of a vehicle body.

The side airbag 12 deployed between a waist section Ph and the chest section Pc of an occupant P seated on the top of a seat 15 and a side door 21 is partitioned into an upper bag 12a and a lower bag 12b. On the upper bag 12a, a point-sewn section 19 is formed to form a variable-displacement section 20. During the deployment of the side airbag 12, the point-sewn section 19 is broken when the inner pressure of the upper bag 12a increases beyond a prescribed pressure because of significant breakage of the side door 21, so that the variable-displacement section 20 is increased with the internal volume of the upper bag 12a and the inner pressure is reduced, thus protecting the chest section Pc.