A side airbag apparatus is provided in a side portion of a vehicle seat back located on the vehicle outer side. The side airbag apparatus includes an airbag unit elongated in the up-down direction and a plate fixed to the airbag unit. The airbag unit has an airbag and an inflator. The plate has a fixing portion and a rear wall portion. The inner side surface of the airbag unit is fixed to the fixing portion. The rear wall portion covers an upper half portion of the airbag unit. The rear wall portion also partly covers the rear surface of the lower half portion of the airbag unit. The plate is made of resin and has flexibility and elasticity.