A heater/cooler unit of an automotive air conditioning device comprises a casing which has air inlet and outlet openings. An air mixing chamber is located just upstream of the air outlet opening. A first air flow passage extends from the air inlet opening to the air mixing chamber. A second air flow passage extends from a downstream part of the first air flow passage to the air mixing chamber. A third air flow passage extends downstream from the air mixing chamber. An evaporator is installed in an upstream part of the first air flow passage for cooling air passing therethrough. A heater core is installed in the second air flow passage for warming air passing therethrough. A mix door is arranged to control the proportion between the amount of air directly led to the mixing chamber from the first air flow passage and the amount of air directed to the second air flow passage from the first air flow passage. A ventilation door is arranged to control the proportion between the amount of air directed from the air mixing chamber to the air inlet opening and the amount of air directed from the air mixing chamber toward the third air passage. The evaporator and the heater core are arranged generally in parallel with each other and arranged to define therebetween an upstream portion of the first air flow passage.