The knee-protecting airbag device includes an airbag that protrudes rearward from a housing and deploys upward when fed with inflation gas. The airbag includes an upstream part located toward lower end of the completely inflated airbag and located upstream of inflation gas, and a downstream part located toward upper end of the completely inflated airbag and located downstream of inflation gas. The downstream part is adapted to protect an area from shins to knees of an occupant when the airbag is completely inflated. In the course of airbag inflation, internal pressure of a shin-protecting area, which is part of the downstream part for protecting shins, peaks 5 to 15 ms later than a peak time of internal pressure of the upstream part.