A rear seat side airbag device includes a cover, a case that is attached to a back side of the cover and a comb teeth-like holding member and is formed by assembling these constituent components together. A breakable portion is formed at a position on the cover which corresponds to the case. In this breakable portion, a first thin portion, which constitutes a breakage initiating position, is formed so as to extend in a vertical direction along a corner portion on a side of the cover which faces a rear seat. In addition, to correspond to the position of the first thin portion, the case is provided such that a first side wall on which a plurality of fragile portions are formed is positioned on a side thereof which faces the rear seat and a second side wall on which a rigid portion is provided is positioned on a side thereof which faces a door trim.