A heat pump system for a vehicle, capable of working in a heat pump mode and in a air-conditioning mode, comprising: a compressor for compressing and discharging a refrigerant; an evaporator mounted inside an air-conditioning case for exchanging heat between the air inside the air-conditioning case and the refrigerant; an exterior heat exchanger mounted outside the air-conditioning case for exchanging heat between the refrigerant and the outdoor air; expansion means arranged between the evaporator and the exterior heat exchanger for expanding the refrigerant; a waste heat exchanger for exchanging heat between waste heat of the vehicle and the refrigerant; a first refrigerant circulation line which connects a refrigerant line in such a manner that refrigerant discharged from the compressor circulates the exterior heat exchanger, the expansion means, the evaporator and the compressor in the air-conditioning mode; and a second refrigerant circulation line which connects a refrigerant line in such a manner that refrigerant discharged from the compressor circulates the evaporator, the expansion means, the waste heat exchanger and the compressor in the heat pump mode; the first refrigerant circulation line and the second refrigerant circulation line using in common a section (a) to which the compressor is connected and a section (b) to which the evaporator and the expansion means are connected; wherein a first direction-changing valve for changing the flow direction of the refrigerant is mounted at a point where the first and second refrigerant circulation lines (R1,R2) are branched at an outlet side of the compressor, such that the refrigerant discharged from the compressor flows toward the first refrigerant circulation line or the second refrigerant circulation line according to the air-conditioning mode or the heat pump mode, and wherein a second direction-changing valve for changing the flow direction of the refrigerant is mounted at a point where the first and second refrigerant circulation lines (R1,R2) are branched at the evaporator inlet side when in the heat pump mode, such that the refrigerant discharged from the evaporator flows toward the compressor along the first refrigerant circulation line in the air-conditioning mode and the refrigerant discharged from the compressor flows toward the evaporator along the second refrigerant circulation line in the heat pump mode, the evaporator being used commonly to carry out a cooling action in the air-conditioning mode and to carry out a heating action, as a condenser, in the heat pump mode. a compressor for compressing and discharging a refrigerant; an evaporator mounted inside an air-conditioning case for exchanging heat between the air inside the air-conditioning case and the refrigerant; an exterior heat exchanger mounted outside the air-conditioning case for exchanging heat between the refrigerant and the outdoor air; expansion means arranged between the evaporator and the exterior heat exchanger for expanding the refrigerant; a waste heat exchanger for exchanging heat between waste heat of the vehicle and the refrigerant; a first refrigerant circulation line which connects a refrigerant line in such a manner that refrigerant discharged from the compressor circulates the exterior heat exchanger, the expansion means, the evaporator and the compressor in the air-conditioning mode; and a second refrigerant circulation line which connects a refrigerant line in such a manner that refrigerant discharged from the compressor circulates the evaporator, the expansion means, the waste heat exchanger and the compressor in the heat pump mode; the first refrigerant circulation line and the second refrigerant circulation line using in common a section (a) to which the compressor is connected and a section (b) to which the evaporator and the expansion means are connected;