A side airbag device includes an airbag 2 and an inflator. The airbag is formed in a bag shape by connecting peripheries of first and second main base cloths and of a vehicle inner side and a vehicle outer side. The airbag is partitioned into a front chamber and a rear chamber by a first partition wall. First inner tethers are provided in the front chamber and connect between a forward vehicle direction end of the front chamber and the first partition wall. Lengths of the first inner tethers and are shorter than a length between a first joint part connecting the first partition wall to the first inner tether and the forward vehicle direction end of the front chamber in the airbag in a front to rear vehicle direction before the airbag is deployed.