To provide a side airbag device capable of obtaining high occupant protection performance by increasing thickness of an inflated airbag in the deployed state without causing a delay in inflation deployment and complicated manufacturing processes.

The side airbag 150 configuration includes: an inflator 154 for feeding a gas for inflation deployment; an airbag 152 that is inflated by the gas for inflation deployment from the inflator so as to be deployed between a vehicle interior lateral surface part (side door) and an occupant toward a predetermined inflation deployment direction; and a strap 156 that has a length shorter than an overall length L1 in the inflation deployment direction of the pre-folded or pre-wound airbag and is connected to both end parts in the inflation deployment direction of the airbag at both ends thereof respectively. The strap is disposed at a surface on a side of the vehicle interior lateral surface part of the airbag, and the airbag is configured so that the surface on the side of the vehicle interior lateral surface part comes into contact with the vehicle interior lateral surface part when the airbag is in the inflating deployment state.