An air conditioner comprising: a first outdoor unit provided with a first compressor, a first outdoor heat exchanger, and a first outdoor expansion valve; and a plurality of indoor units provided with indoor heat exchangers and connected by an inter-unit pipeline, one end of the first outdoor heat exchanger being selectively branched and connected to a refrigerant discharge pipe and a refrigerant sucking pipe of the first compressor, the inter-unit pipeline including a high-pressure gas pipe connected to the refrigerant discharge pipe, a low-pressure gas pipe connected to the refrigerant sucking pipe, and a fluid pipe connected to the other end of the first outdoor heat exchanger, one end of the indoor heat exchanger being selectively branched and connected to the high-pressure gas pipe and the low-pressure gas pipe, and the other end of the indoor heat exchanger being connected to the fluid pipe through a fluid branch pipe so that the plurality of the indoor units can perform a cooling operation or a heating operation at the same time or the cooling operation and the heating operation can be performed in a mixed manner, wherein a second outdoor unit provided with a second compressor, a second outdoor heat exchanger, and a second outdoor expansion valve and connected by two pipelines of a gas pipe and a fluid pipe, the fluid pipe of the second outdoor unit being connected to the fluid pipe of the inter-unit pipeline, and the gas pipe of the second outdoor unit being selectively connected to the high pressure gas pipe or the low-pressure gas pipe of the inter-unit pipeline using a valve-element kit having a channel switching valve, characterized in that the valve-element kit is provided with a single four-way valve as the channel switching valve, in which the gas pipe is connected to a first port of this four-way valve, the low-pressure gas pipe is connected to a second port, the high-pressure gas pipe is connected to a third port, and a fourth port is closed or the low-pressure gas pipe is connected to the fourth port through a capillary tube. a first outdoor unit provided with a first compressor, a first outdoor heat exchanger, and a first outdoor expansion valve; and a plurality of indoor units provided with indoor heat exchangers and connected by an inter-unit pipeline, one end of the first outdoor heat exchanger being selectively branched and connected to a refrigerant discharge pipe and a refrigerant sucking pipe of the first compressor, the inter-unit pipeline including a high-pressure gas pipe connected to the refrigerant discharge pipe, a low-pressure gas pipe connected to the refrigerant sucking pipe, and a fluid pipe connected to the other end of the first outdoor heat exchanger, one end of the indoor heat exchanger being selectively branched and connected to the high-pressure gas pipe and the low-pressure gas pipe, and the other end of the indoor heat exchanger being connected to the fluid pipe through a fluid branch pipe so that the plurality of the indoor units can perform a cooling operation or a heating operation at the same time or the cooling operation and the heating operation can be performed in a mixed manner, wherein a second outdoor unit provided with a second compressor, a second outdoor heat exchanger, and a second outdoor expansion valve and connected by two pipelines of a gas pipe and a fluid pipe, the fluid pipe of the second outdoor unit being connected to the fluid pipe of the inter-unit pipeline, and the gas pipe of the second outdoor unit being selectively connected to the high pressure gas pipe or the low-pressure gas pipe of the inter-unit pipeline using a valve-element kit having a channel switching valve, characterized in that the valve-element kit is provided with a single four-way valve as the channel switching valve, in which the gas pipe is connected to a first port of this four-way valve, the low-pressure gas pipe is connected to a second port, the high-pressure gas pipe is connected to a third port, and a fourth port is closed or the low-pressure gas pipe is connected to the fourth port through a capillary tube.