A side airbag device includes an inflator and an airbag. The airbag includes a deployable bag and a separator that separates the bag into a first chamber and a second chamber. The first chamber includes a support inflation portion and a protection inflation portion. The support inflation portion is deployable in such a shape that it gradually broadens from its lower end portion toward its upper end portion along a seat-side portion. The protection inflation portion is deployable at one end side of the support inflation portion. The inflator is disposed to supply a gas into the support inflation portion. The separator includes at least one gas distribution hole in a portion thereof that separates the support inflation portion of the first chamber and the second chamber from each other. The second chamber is provided below the first chamber.