A side airbag device capable of preventing interference between a harness and an airbag is provided between a vehicle body and a seat. The side airbag device includes an airbag module including an inflatable airbag and an inflator supplying a gas into the airbag, a base member including an airbag storage portion having a concave shape and storing the airbag module and is attached between a vehicle body door and the seat, and a retainer member holding the airbag storage portion from the rear side of the vehicle body. The base member includes a first through hole through which a harness of the inflator is inserted and a flange which is formed in the periphery of the first through hole. The retainer member includes a second through hole through which the harness of the inflator is inserted and which is provided at a position facing the first through hole.