A curtain air bag apparatus for protecting an occupant in a vehicle upon a vehicle collision, has an air bag body inflating and deploying in a curtain-like shape in a side surface of a vehicle interior with a high pressure gas jetted and introduced into the air bag body by an inflator. The air bag body has a main inflation chamber and a sub-inflation chamber which are defined by a partition portion formed by bonding inside surfaces of the air bag body, and a gas flow regulation passage formed as a partly non-bonding portion of the partition portion. The main and sub-inflation chambers are respectively located at an upper side and a lower side of the air bag body and communicate with each other through the gas flow regulation passage.