The present invention provides a side curtain air bag facilitating the control of tension with the air bag. The side curtain air bag includes chambers expanded by gas supplied from a gas generator, and expands and develops into a curtain shape along a side part of a vehicle so as to protect vehicle occupants. The side curtain air bag includes primary chambers which expand to form the side curtain air bag, and a secondary chamber which expands after the primary chambers to apply additional tension across the expanded side curtain air bag. The secondary chamber includes an opening communicating with the primary chamber.