A side airbag apparatus includes an airbag that can form a plurality of chambers with a single inflator and protects an occupant from an impact on a side door by deploying the airbag between the occupant and the side door. The chambers of the airbag includes a vehicle sidewall chamber that can be deployed around a shoulder region of the occupant and a compartment chamber that is disposed closer to the inner side of an occupant compartment in the vehicle width direction than the vehicle sidewall chamber and can be deployed around a chest region of the occupant.