A heat pump system including a compressor mounted on a refrigerant circulation line for compressing and discharging refrigerant. An internal heat exchanger is mounted inside an air-conditioning case. An evaporator is mounted inside the air-conditioning case. An external heat exchanger is mounted outside the air-conditioning case. A first expansion means is mounted on the refrigerant circulation line between the internal heat exchanger and the external heat exchanger to selectively expand refrigerant discharged from the internal heat exchanger. A second expansion means is mounted on the refrigerant circulation line of an inlet side of the evaporator to expand the refrigerant supplied to the evaporator. A coolant circulation line is configured to circulate coolant toward electronic units of the vehicle. A refrigerant-coolant heat exchanger is configured to exchange heat between the refrigerant flowing the refrigerant circulation line and the coolant circulating through the coolant circulation line.