A side airbag apparatus is provided with an inflator, and an airbag. The airbag is stored in a vehicle seat, is inflated and deployed by gas from the inflator, breaks the vehicle seat so as to jump out, and is inflated and deployed between a body side portion of a vehicle and the vehicle seat. In this side airbag apparatus, a controller predicts a side collision of the vehicle, and makes the inflator start an actuation in advance of the side collision in response to the prediction. Further, in the airbag inflated and deployed in accordance with the actuation start of the inflator, a deploying speed of the airbag is lowered in comparison with the deploying speed at a time of starting the actuation of the inflator after the side collision, at least outside the vehicle seat.