In the head-protecting airbag device according to the present invention, an airbag folded and housed in the upper edge of an opening inside the vehicle deploys downward to cover the opening. The airbag includes a plurality of cells juxtaposed in the front-rear direction each of which, when fed with inflation gas, extends downward from the upper edge of the opening and inflates to separate its interior and exterior walls. Above the cells is located a gas passage extending in the front-rear direction, via which the cells are communicated with one another. The gas passage includes a joint port for introducing inflation gas into the airbag. In each of the cells, the interior wall and exterior wall are respectively bellows-folded on alternate crest folds projecting in the surface and valley folds recessing inside the cell, such that the airbag is folded to bring its lower edge close to its upper edge. The head-protecting airbag device is able to deploy the airbag swiftly.