A side airbag device has: a side airbag that inflates and expands at a time of a side collision, and that has a lower chamber (high pressure chamber), and an upper chamber (low pressure chamber), the high pressure chamber and the low pressure chamber being formed by base cloths (a lower base cloth, an upper base cloth) that are respectively separate bodies; an inflator; a partitioning wall that is formed by a portion of the lower base cloth, and that divides the lower chamber and the upper chamber; and a check valve that is provided at a partial region of the partitioning wall, and that is provided convexly toward the lower chamber side when the side airbag is not inflated, and that permits flowing of the gas for inflation from the upper chamber side to the lower chamber side, and restricts flowing of gas in a direction opposite thereto.