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 (4A-4D) connected to the first outdoor unit by an inter-unit pipeline and provided with indoor heat exchangers (10A-10D), 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 (18A-18D) 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, characterized in that 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 and a valve-element kit having a channel switching valve that connects the fluid pipe of the second outdoor unit to the fluid pipe of the inter-unit pipeline and selectively connects the gas pipe of the second outdoor unit to the high-pressure gas pipe or the low-pressure gas pipe of the inter-unit pipeline are provided; the first outdoor unit is provided with a valve element that makes the refrigerant discharge pipe and the high-pressure gas pipe capable of communicating with each other; and in a case of the cooling operation of the indoor units at the same time, the valve element shuts off the communication between the refrigerant discharge pipe and the high-pressure gas pipe and the channel switching valve is switched so as to connect the gas pipe of the second outdoor unit to the high-pressure gas pipe of the inter-unit pipeline. 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 (4A-4D) connected to the first outdoor unit by an inter-unit pipeline and provided with indoor heat exchangers (10A-10D), 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 (18A-18D) 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, characterized in that 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 and a valve-element kit having a channel switching valve that connects the fluid pipe of the second outdoor unit to the fluid pipe of the inter-unit pipeline and selectively connects the gas pipe of the second outdoor unit to the high-pressure gas pipe or the low-pressure gas pipe of the inter-unit pipeline are provided; the first outdoor unit is provided with a valve element that makes the refrigerant discharge pipe and the high-pressure gas pipe capable of communicating with each other; and in a case of the cooling operation of the indoor units at the same time, the valve element shuts off the communication between the refrigerant discharge pipe and the high-pressure gas pipe and the channel switching valve is switched so as to connect the gas pipe of the second outdoor unit to the high-pressure gas pipe of the inter-unit pipeline.