To promote the miniaturization of an automobile air conditioner, and also to increase its volumetric capacity in a cabin.

In this heater unit of an automobile air conditioner equipped with a heater core 5 for heating a flow of air passed through an evaporator, an air-mix door 4 regulating the rate of air passing through this heater core 5 and another air passing through the evaporator, and a heater unit case 18 storing both of the heater core 5 and this air-mix door 4, the heater core 5 is almost perpendicularly set up so as to make the air from the evaporator flow in front of the car from the rear. With this setup, a car longitudinal width of the heater unit case 18 is reduced.