Abstract: An air conditioner for a vehicle has an air conditioning casing for defining therein a passage of air, a heating heat exchanger for heating air, and an air mixing door at an air upstream side of the heating heat exchanger to adjust an air volume ratio between air passing the heating heat exchanger and air passing a cool air bypass passage. In an air mixing state where the air mixing door is positioned at a substantially intermediate position between a maximum bypass position and a maximum heating position, additional air flow is compulsorily caused at a generation end of an interface between cool air and warm air in a cool-warm air mixing space, where cool air supplied through the cool air bypass passage and warm air heated by the heating heat exchanger and supplied through a warm air passage are mixed.