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 inflated and 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 an inner portion of the primary airbag in the vehicle width direction, communicating with the primary airbag through a first communication hole, and configured to be inflated and deployed inward in the vehicle width direction from the primary airbag. The first communication hole is at least partially arranged more forward than a front surface of the outer portion of the seat back in an inflated and deployed state of the primary airbag. The secondary airbag comes in contact with a front surface of the seat back in an inflated and deployed state of the secondary airbag.