A side airbag apparatus has an inflatable side airbag, an inflator which supplies the side airbag with gas, a base member in which the folded side airbag and inflator are accommodated, and a harness connected to the inflator to supply it with power for ignition. The base member has a backside surface which faces an inside of the seat, a frontside surface, a protrusion portion formed on the frontside surface, and a harness supporting portion below the protrusion portion which fixes a portion of the harness so as to restrict lateral and vertical movements of the harness relative to the frontside surface.