A side airbag (SAB) of a vehicle includes: a main chamber disposed in a seat to be deployed from a side surface portion of the seat and, when deployed, configured to cover at least one of a thoracic region, an abdomen region, or a pelvis region of an occupant; a main inflator configured to supply gas to the main chamber to deploy the main chamber; and an auxiliary chamber communicating with the main chamber to allow the gas inside the main chamber to be supplied to an interior of the auxiliary chamber and connected to the main chamber to be deploy above the main chamber.