A side airbag apparatus includes a side airbag housed in a seat back of a seat of a vehicle. The side airbag includes a lower protection portion configured to protect a portion below a head portion of the occupant, and an upper protection portion configured to protect the head portion. The side airbag is folded in a state in which the upper protection portion is folded toward a lower side so as to be positioned on an outer side in a width direction of the vehicle with respect to the lower protection portion, and a portion, that protrudes toward the front side with respect to the lower protection portion, of the upper protection portion folded toward the lower side, is folded toward a rear side so as to be positioned on an inner side in the width direction with respect to the lower protection portion.