In the side airbag device, in an inflation and deployment state of the side airbag, a tension cloth extendingly disposed across between a front-rear partition and a rear end of the rear bag section is stretched, thereby restricting inflation in the vehicle front-rear direction of the rear bag section. An inflating width in a vehicle width direction of the rear bag section can be increased without significantly changing a position in the vehicle front-rear direction of the front-rear partition. In a flat developed state of the side airbag, the tension cloth is extendingly disposed across between the front-rear partition so folded into two as to protrude toward the rear bag section, and the rear end of the rear bag section. Accordingly, dimensions of each component can be so defined as to cause no redundancy in the rear bag section in the flat developed state, thus compactly folding the side airbag.