Described is an apparatus for air conditioning a passenger compartment of an automotive vehicle which achieves a quick warming-up of the passenger compartment. In the air conditioning apparatus, in addition to a coolant of a vehicular engine which is utilized as a heat source of a heater core, a highly compressed refrigerant discharged from a compressor can be utilized as a heat source of a sub condenser which is disposed in an air passage of the air conditioning apparatus, during the inside air warming up in the passenger compartment. During the warming up operation, the refrigerant discharged from the compressor is circulated through a bypass line bypassing a condenser disposed at a front part of an engine compartment and through the sub condenser and an evaporator.