A side airbag device is provided in a seat back of a vehicle seat. The side airbag device includes: a primary airbag configured to be deployed forward and in an up-and-down direction from an outer portion of the seat back in a vehicle width direction; and a secondary airbag connected to the primary airbag via a connecting portion, communicating with the primary airbag through a communication hole provided in the connecting portion, and configured to be deployed in the vehicle width direction from the primary airbag. The secondary airbag is folded to a side opposite to the primary airbag along a first fold passing above the connecting portion and extending in a front-and-rear direction. In a state where the secondary airbag is folded along the first fold, the primary airbag and the secondary airbag are wound or folded around an axis extending in the up-and-down direction.