A vehicle seat equipped with a side airbag device includes: an inflator disposed on an outer side in a vehicle width direction relative to a side frame; an inner chamber housed inside the side part, and is configured to inflate and deploy toward a part of an occupant on the outer side in the vehicle width direction upon receiving supply of a gas from the inflator; an outer chamber housed inside the side part, and is configured to inflate and deploy toward the outer side in the vehicle width direction and a vehicle front side relative to the inner chamber upon receiving supply of the gas; and a diffuser housed inside the side part and configured to supply the gas into the inner chamber from a first supply port formed in a peripheral wall of the diffuser when the inflator is activated.