An air bag of an inflatable curtain device includes a first cell group and a second cell group whose internal spaces are independent from each other. A high-pressure gas discharged from opposite ends of a common inflater disposed between both of the first and second cell groups and is supplied to a high-pressure gas supply port in the first cell group and a high-pressure gas supply port in the second cell group. The first and second cell groups can be expanded simultaneously by the common inflater, whereby the entire air bag can be deployed promptly, but also even when one of the first and second cell groups and is damaged, the deployment of the other cell group can be achieved without hindrance.