SIDE AIRBAG DEVICE

A side airbag device comprising a bag-like cushion that is provided to a side section of a seat back of a vehicle seat and inflates and expands toward the side of a passenger in the vehicle seat using a gas. The cushion includes: a first part having four sides and including a first chamber; a second part that has four sides, is adjacent to the first part, and includes a second chamber; and a third part that has four sides, is adjacent to the first part and the second part, and does not inflate. When inflated and expanded, the cushion is a triangular column that is a columnar shape having a space E open on both the top and bottom and surrounded by the first part in which the first chamber is inflated and expanded, the second part in which the second chamber is inflated and expanded, and the non-inflated third part.