A head-protecting airbag apparatus includes an airbag having such a vertical dimension that the lower end region thereof is located on the beltline of a door of a vehicle upon deployment. The airbag includes a plurality of vertical cells disposed side by side along an anteroposterior direction and each inflatable into a rod-like shape extending generally vertically and a reinforcing portion located either on the inboard side or on the outboard side of at least two adjoining vertical cells out of the vertical cells at the lower end region of the airbag. The reinforcing portion is in gas communication with the vertical cells and inflatable into a horizontal rod-like shape (extending across the adjoining vertical cells).