In a knee-protecting airbag device according to the present invention, an airbag folded and housed in undeployed condition protrudes rearward from a housing case to deploy upward and protects knees of a driver upon inflow of inflation gas. The airbag includes a cover portion for covering a vehicle's rear side of an area proximate to the key cylinder upon completion of deployment, and a general portion in the upper edge side of the airbag except the cover portion. Kinetic energy of the cover portion as deployed in a position of complete deployment is restrained compared with a kinetic energy of the general portion as deployed in a position of complete deployment. This restraint is achieved by reducing amount of inflation gas flowing into the cover portion. The knee-protecting airbag device of the present invention prevents the inflating airbag from contacting with key accessories hung down from an ignition key inserted into the key cylinder in a manner to cause gas leakage.