A side airbag apparatus includes an airbag, an inflator and a limited thickness portion. The inflator supplies gas to the airbag. When the airbag is inflated and deployed, the limited thickness portion limits the inflation of the airbag such that the thickness of a portion of the airbag corresponding to an elbow portion of an occupant on the seat is less than the thickness of the remainder of the airbag with respect to the measurement between the occupant and the body side portion. The airbag includes an inflation portion that entirely surrounds the limited thickness portion and is inflated with gas from the inflator. This prevents an elbow portion of the occupant from receiving an excessive force directed laterally inward.