The airbag unit is disposed inside the rear upper face portion of the instrument panel near the difference-in-level portion. The lid portion is configured to open substantially forward around the hinge member without hitting against the difference-in-level portion. Herein, the hinge member is configured such that the lid portion can open beyond the difference-in-level portion to the position where the lid portion is substantially located on the front upper face portion of the instrument panel when the airbag is inflated to its maximum opening state. Accordingly, the proper protection of the passenger seated in the assistant seat can be performed.