To solve a problem that when an air conditioner is used at cold start, cold outside air passes through a heater core, leading to a delay of a warming-up time of an engine.

An air conditioner for an automobile includes: a ventilation duct having a suction port on one end side and an air blowout port to inside of a cabin on the other end side; a blower arranged on the suction port side within the ventilation duct; a heater core disposed on the blowout port side from the blower; and a mixing door located between the blower and the heater core and movable in accordance with a set temperature. A shielding member for blocking air passing through the heater core in accordance with a cooling water temperature is installed on the outer face of the heater core. In the air conditioner, until warming-up is completed, passage of cold air through the heater core is prevented, so as to enable prompt warming-up.