[Problem]
To provide: a side airbag device capable of quickly and properly restraining a passenger in the initial stage of deploying an airbag; and a vehicle seat provided with the side airbag device.
[Resolution Means]
A side airbag device is provided with: an airbag disposed inside a side supporting part of a seat, that restrains an occupant by expanding and deploying in the event of a vehicle collision; and an inflator that supplies an expansion gas to the airbag. Furthermore, the airbag is folded and stored in the side supporting part in an extended condition such that the vertical direction is the longitudinal direction. Furthermore, a lower part of the airbag in the stored condition is folded a plurality of times in the longitudinal direction to form a lower compressed part.