In an air bag unit for use in a motor vehicle, including a casing mounted onto a steering wheel assembly, a gas generator arranged in the casing, an inflatable air bag fixed to the casing and communicating with the gas generator and a cover adapted to cover the casing, the cover comprises a base portion fixedly mounted to the casing and a top portion connected to the base portion and having first and second weakened portions, the first and second weakened portions being broken upon inflation of the air bag, so that the top portion is divided into two cover pieces and that these cover pieces are open outwardly, a reinforcing member embedded in the cover except the first and second weakened portions, and one or a plurality of hooks arranged in the base portion, each hook being engaged with the reinforcing member and being fixed to the casing of the air bag unit. Each hook includes a base plate and a bent portion. The base plate is fixed to the casing, and the bent portion has a plurality of hook elements with which the reinforcing member is engaged.