An automotive air conditioning system has a primary hot water circuit 11 located on a side where a vehicle installed heat generator 10 is located and a secondary hot water circuit 13 which includes a hot water type heater core 12 for heating passenger compartment outlet air, whereby when in a heating mode, in the event that a coolant temperature TW1 of the primary hot water circuit is lower than a coolant temperature TW2 of the secondary hot water circuit 13, an opening and closing valve 26 is closed, whereas an opening and closing valve 23 is opened, so that a state is created in which the hot water circuits 11, 13 are separated from each other. On the other hand, in the event that the coolant temperature TW1 of the primary hot water circuit becomes higher than the coolant temperature TW2 of the secondary hot water circuit 13, the opening and closing valve 26 is opened, whereas the opening and closing valve 23 is closed, so that a state is created in which the hot water circuits 11, 13 are connected to each other.