A head-protecting airbag device according to the present invention includes an airbag, a roof rail garnish formed into a single plate shape for covering the interior of the folded airbag, and being provided in its lower edge with a door portion to be pushed by the inflating airbag, and an inflator. These members are assembled into an airbag module and mounted on a vehicle as a single part. The airbag module includes a cover portion located in a back side of the roof rail garnish over the generally entire length of the folded airbag. The cover portion includes a bottom wall extending from an exterior side of the door portion for covering a lower side of the folded airbag, and a side wall for covering an exterior side of the airbag. An upper part of the side wall of the cover portion is joined to an exterior portion of the garnish above the folded airbag. The head-protecting airbag device of the present invention is able to protect the airbag until it is mounted on the vehicle, and is also able to suppress a protruding amount of the roof rail garnish toward the interior when in service.