A side airbag device is provided, which is capable of increasing an energy absorption amount while suppressing the load on an occupant. The side airbag device includes: a bag-shaped cushion; an inflator provided on a vehicle rear side inside the cushion; an inner tube that surrounds the inflator; an upper opening and a lower opening formed in the inner tube; a tether that extends toward a vehicle front side from the inner tube to partition the vehicle front side of the inside of the cushion into a chest restraining portion and a waist restraining portion; a void portion provided inside the cushion on the vehicle front side of the tether; and a vent hole formed near the void portion. The upper opening has a smaller opening area than the lower opening, and the tether is inclined downward toward the vehicle front side from the vehicle rear side.