a front seat air conditioning unit having a front seat air conditioning casing provided with a front seat evaporator and a front seat heater core disposed on air flow passages therein, with vent outlets, defrost outlets and floor outlets disposed at an exit end thereof for discharging the air flowing to the front seats of the vehicle, with a front seat partition wall for dividing the air flow passages into two areas, with front seat temperature doors for individually controlling the temperatures of the two areas divided by the front seat partition wall, with front mode doors for controlling the degree of opening of the vent outlets, the defrost outlets and the floor outlets and with the front seat blower for blowing the air to the front seat air conditioning casing; and a rear seat air conditioning unit having a rear seat air conditioning casing provided with a rear seat evaporator disposed in substantially in the horizontal direction to air flow passages therein, with a rear seat heater core disposed on the upper side of the rear seat evaporator, with vent outlets and floor outlets disposed at an exit end thereof for discharging the air flowing to the rear seats of the vehicle, with a rear seat partition wall for dividing the air flow passages into two areas, with rear seat temperature doors disposed between the rear seat evaporator and the rear seat heater core, for individually controlling the temperatures of the divided areas by the rear seat partition wall, with rear seat mode doors disposed adjacent to the vent outlets and the floor outlets for controlling the degree of opening of the vent outlets and the floor outlets, and with a rear seat blower for blowing the air to the rear seat air conditioning casing, whereby in a heating mode, air flows into the upper side portion of the rear seat heater core while passing through the rear seat heater core so as to be discharged to the rear seats of the vehicle.