Vehicular heat management system includes a refrigerant circulation line including a compressor, a high pressure side indoor heat exchanger, a heat pump mode expansion valve, an outdoor heat exchanger, an air conditioning mode expansion valve, low pressure side indoor heat exchanger and a refrigerant-cooling water heat exchanger; an air conditioning mode branch line branched from the refrigerant circulation line and configured to, in an air conditioning mode, form a cooling loop by circulating a refrigerant through the compressor, the high pressure side indoor heat exchanger, the outdoor heat exchanger, the air conditioning mode expansion valve and low pressure side indoor heat exchanger. Heat pump mode branch line branched from the refrigerant circulation line is configured, in heat pump mode to form a heating loop by circulating the refrigerant through the compressor, the high pressure side indoor heat exchanger, the heat pump mode expansion valve and the refrigerant-cooling water heat exchanger.