Cooled air introduced from an evaporator through a first passage is divided into laminar flows by a guide assembly mounted on an air mixing door, and is changed in direction thereby and supplied to a mixing region. Warmed air introduced from a heater core through a third passage is divided into laminar flows by the guide assembly, and is changed in direction thereby and supplied to the mixing region. The mixing region mixes the cooled air and the warmed air with each other, and delivers uniformly mixed air into a passenger compartment via a face outlet, a defroster outlet, or a foot outlet.