The present invention relates to a vehicular heat pump system and, more specifically, to a vehicular heat pump system having a chiller for heat exchanging cooling water circulating through vehicle electrical equipment (200), a refrigerant discharged from an outdoor heat exchanger so as to flow through a first bypass line (R1), and a refrigerant discharged from an indoor heat exchanger, thereby: in the air conditioner mode, increasing sub-cool while the refrigerant discharged from the indoor heat exchanger is first cooled at the chiller and then is second cooled at the outdoor heat exchanger such that the flow rate of refrigerants increases and cooling performance is improved; in the heat pump mode, allowing the refrigerant discharged from the indoor heat exchanger to ensure sub-cool in the chiller so as to increase the flow rate of refrigerants, and allowing the refrigerant discharged from the outdoor heat exchanger to receive an additional heating source from the chiller so as to improve heating performance; and, even in the defrost mode carried out because of frost on the outdoor heat exchanger, ensuring an additional heating source in the chiller so as to minimize a change in indoor discharge temperature because of a lack of heating sources.