Cooled air introduced from an evaporator through a first passage in a housing is divided into laminar flows by a guide structure disposed on an air mixing door. The direction of flow of the cooled air is changed upwardly by guide walls of the guide structure and is further changed upwardly by first walls of the guide structure, after which the cooled air is introduced into a mixer. Heated air introduced from a heater core through a third passage is mixed with the cooled air in the mixer. The mixed air is delivered from a face discharge outlet, a defroster discharge outlet, or a foot discharge outlet into a passenger compartment.