A side airbag device 1 having a tensioned tether 5 is attached to an outer surface of the airbag 2 on a side facing an occupant. The tether 5 has a shape with a front edge 5a vertical length a longer than a rear edge 5b vertical length. The front edge 5a continuously extends in a vertical direction along a front end 2c of the airbag 2 between upper and lower parts when deployed. The rear edge continuously extends in the vertical near a rear end 2d of the airbag 2 between the upper and lower parts. The front edge 5a joins in the vertical direction to the airbag 2 front end 2c, and the tether 5 moves in an oblique direction having a vertical and horizontal direction components in a state of being pulled in an oblique direction inclined to the vertical direction when deployed.