Abstract: An air conditioner for an automobile, characterized by having an opening of a defrost duct for guiding air toward the inner face of a windshield formed in an upper portion of a heater unit, an opening of a foot duct for guiding air toward an automobile occupant's foot rest formed in a lower portion of said heater unit, a heater core incorporated in a central portion of said heater unit, a first mixing chamber formed in the proximity of said opening of said defrost duct and second mixing chamber formed in the proximity of said opening of said foot duct both within said heater unit, a first mix door for adjusting the amount of air flowing into said heater core and the amount of air bypassing said heater core and directly flowing into said first mixing chamber attached rotatably to the front side of an air inlet face of said heater core, a second mix door for adjusting the amount of air flowing through said core heater into said second mixing chamber and the amount of air bypassing said heater core and flowing d