The knee-protecting airbag apparatus (M1) includes an airbag (61) folded and housed in a lower area (16a) inside the column cover (16). The airbag (61) pushes and opens a door section (34) on the column cover upon deployment. When the door section (34) is thus opened, a door (35) of the door section (34) blocks off an allowance gap (H) formed between an outer circumference (18) of the column cover (16) subjected to position adjustment and a lower area (11a) of a peripheral region (11) of an installation opening (10) on a dashboard (7) for installing the column cover (16) by locating a rear edge region (36) of the door (35) to abut on a rear side (12) of the dashboard (7) or the lower area (11a) of the peripheral region (11) of the installation opening (10) on the rear side (12) of the dashboard (7).