A knee-protecting airbag device is located below a steering column in front of a driver and, at a lower side of a key cylinder into which an ignition key with accessories is inserted. The airbag device includes a housing located below the steering column for housing an airbag and an airbag. The airbag is folded and housed in the housing in undeployed condition, and upon inflow of inflation gas, protrudes rearward from the housing to deploy upward to protect knees of the driver. The airbag is provided in its upper edge in completely deployed condition with a cover portion for covering vehicle's rearward of an area proximate to the key cylinder. The airbag is further provided with a flexible reinforcing cloth to be located in a part of the cover portion to contact the key accessories as the key is inserted into the key cylinder. The airbag is protected by the reinforcing cloth even if it contacts with the key accessories upon deployment.