An air conditioner comprising: a first outdoor unit including an engine operated by combustion of a gas fuel, a compressor connected to the engine by a belt for compressing refrigerant, and a first casing exposed outside and providing a space where the engine and the compressor are placed; an indoor unit including an indoor heat exchanger for transferring heat between air in an indoor space and refrigerant discharged from the compressor; a second outdoor unit including an outdoor heat exchanger that transfers heat between outside air and the refrigerant discharged from the compressor, a switching valve configured to send the refrigerant discharged from the compressor to the outdoor heat exchanger or the indoor heat exchanger, and a second casing exposed outside and providing a space where the outdoor heat exchanger and the switching valve are placed; a first refrigerant tube that connects the compressor and the switching valve; a water tube connected to the engine; and a second refrigerant tube that connects the switching valve and the indoor heat exchanger, wherein the second outdoor unit is spaced apart from the first outdoor unit; and the first refrigerant tube, the water tube and the second refrigerant tube are exposed on the outside of the first outdoor unit and second outdoor unit, wherein the first casing comprises a plurality of first peripheral panels placed around four sides of the first casing, wherein the second casing comprises a plurality of second peripheral panels placed around four sides of the second casing, wherein the first refrigerant tube and the water tube have at least one bending portion, characterized in that the plurality of first peripheral panels includes a first service panel where a first connector is placed, with the first refrigerant tube and the water tube being connected to the first connector, and the plurality of second peripheral panels includes a second service panel, where a second connector is placed, with the second refrigerant tube and the water tube being connected to the second connector. a first outdoor unit including an engine operated by combustion of a gas fuel, a compressor connected to the engine by a belt for compressing refrigerant, and a first casing exposed outside and providing a space where the engine and the compressor are placed; an indoor unit including an indoor heat exchanger for transferring heat between air in an indoor space and refrigerant discharged from the compressor; a second outdoor unit including an outdoor heat exchanger that transfers heat between outside air and the refrigerant discharged from the compressor, a switching valve configured to send the refrigerant discharged from the compressor to the outdoor heat exchanger or the indoor heat exchanger, and a second casing exposed outside and providing a space where the outdoor heat exchanger and the switching valve are placed; a first refrigerant tube that connects the compressor and the switching valve; a water tube connected to the engine; and a second refrigerant tube that connects the switching valve and the indoor heat exchanger, wherein the second outdoor unit is spaced apart from the first outdoor unit; and the first refrigerant tube, the water tube and the second refrigerant tube are exposed on the outside of the first outdoor unit and second outdoor unit, wherein the first casing comprises a plurality of first peripheral panels placed around four sides of the first casing, wherein the second casing comprises a plurality of second peripheral panels placed around four sides of the second casing, wherein the first refrigerant tube and the water tube have at least one bending portion, characterized in that the plurality of first peripheral panels includes a first service panel where a first connector is placed, with the first refrigerant tube and the water tube being connected to the first connector, and the plurality of second peripheral panels includes a second service panel, where a second connector is placed, with the second refrigerant tube and the water tube being connected to the second connector.