Side airbag device

A side airbag device disposed in a seat back, the side airbag device comprising:

• an airbag fixed to the seat back in a folded state and configured to be inflated forward from the seat back when the airbag is supplied with a gas;
• and
• a tether configured to restrict a shape of the airbag when the airbag is inflated, wherein
• the airbag includes:
• a first bag portion fixed to the seat back;
• and
• a second bag portion displaced in a direction away from the first bag portion when the airbag is inflated,
• the air bag does not include an inflation portion, which is inflated when receiving the gas, in at least a section between the first bag portion and the second bag portion,
• the tether includes:
• a first tether portion fixed to the first bag portion;
• and
• a second tether portion fixed to the second bag portion,
• a distance between the first tether portion and the second tether portion in a state where the tether is disposed in a planar shape is smaller than a distance between the first bag portion and the second bag portion in a state where the airbag is disposed in a planar shape, and
• the airbag and the tether are configured such that the second bag portion, the second tether portion, the first bag portion, and the first tether portion are positioned inside the seat back in a state where the airbag is deployed.