An airbag 40 includes a pair of fabric main portions 43, 44, which are laid on top of each other in the width direction of a vehicle. The fabric main portions 43, 44 are joined to each other at the periphery. An inflator 31 incorporated in the airbag 40 jets inflation gas G through gas outlet 33, thereby inflating and deploying the airbag 40 forward in a space between an occupant P and a body side portion. A third reinforcing fabric sheet 55 includes a reinforcing fabric portions 57, 58. The reinforcing fabric portions 57, 58 extend to a position forward of a center portion C of the fabric main portions 43, 44 in a front-rear direction from the rear ends of the fabric main portions 43, 44, which are located outside of the reinforcing fabric portions 57, 58. The front ends of the reinforcing fabric portions 57, 58 are joined (sewn) to the fabric main portions 43, 44 by front joint portions 66. The outer reinforcing fabric portion 58 has a hole 67 in a part between a rear end 58R and the front joint portion 66.