The knee protecting airbag device is assembled such that a retainer and an attachment bracket clamp the peripheral portion of a gas inlet opening of an airbag, bag holding wall parts of a case, and a gas providing inflator. This inflator is provided with a diffuser for covering the gas discharge port of a body. The retainer and the attachment bracket are connected by inserting bolts and connecting members through the airbag and the case, by retaining retaining portions on the retaining portions of the connecting members, and by inserting the bolts into through holes and fastening them with nuts. At this connecting time, guide portions of the retainer and the diffuser are fitted on each other to form a guide passage for guiding the gas into the airbag.