The knee protection airbag device (S1) includes an airbag (40) which protrudes rearward from a deployment opening of a housing upon inflow of inflation gas, and deploys upward to be located in front of knees of an occupant between a vehicular body and the occupant. The airbag (40) includes an occupant's side wall (41) and a vehicular body side wall (42). Lower end (48) side of the airbag (40) as flatly expanded with the walls (41 and 42) overlaid on each other constitutes an upstream side of inflation gas. The airbag (40) is firstly folded to bring its upper end (47) close to its lower end (48), from flat expanded state in which the occupant's side wall (41) and the vehicular body side wall (42) are overlaid on each other. Thereafter, the airbag (40) is folded to reduce its transverse width, and then is housed in the housing. The airbag (40) of the knee protection airbag device (S1) is able to deploy effectively from the start to completion of deployment for protecting the knees of the occupant.