A vehicular side airbag apparatus including chambers corresponding to an occupant's shoulder, lumbar, and chest and having three-dimensional shapes formed by baffles inside a side airbag and providing a thickness. The first baffle extends in a vehicular front-rear direction and is joined to a rear edge 103a of a side airbag 103 at a rear end 107b of the first baffle. The second baffle extends in a vehicular up-down direction and has an overlap portion 109 which is set at a longitudinally intermediate position of the second baffle and to which a longitudinally front end 107c of the first baffle is connected along with a vehicular interior panel 105 and a vehicular exterior panel 106. Longitudinal both ends 108b and 108c of the second baffle are joined to an outer peripheral edge of the side airbag 103.