A curtain air bag device which allows a bag main body to expansively spread more quickly and reliably. A curtain air bag device comprises a bag main body received inside a head lining disposed on the window upper edge of a vehicle and designed to expansively spread from the lower end of the head lining toward the interior of the vehicle at the time of actuation; and a gas supply device for supplying expansion gas to the bag main body. The bag main body has a plurality of chambers which expand during operation; and a duct extending above the chambers along the window upper edge to distribute the gas to respective chambers. Further, the duct has an expansion region where the lower end is disposed lower than the lower end of the head lining during spreading.