An air-conditioning apparatus comprising: an outdoor unit ; an indoor unit ; and a relay unit ; a compressor ; a heat source side heat exchanger ; an expansion device ; a heat exchanger related to heat medium ; a pump; and a plurality of use side heat=exchangers , the compressor , the heat source side heat exchanger , the expansion device , and the heat exchanger related to heat medium being connected with two refrigerant pipes to form a refrigerant circuit in which a heat-source-side refrigerant is circulated, the pump, the plurality of use side heat exchangers , and the heat exchanger related to heat medium being connected with heat medium pipes to form a heat medium circuit in which a heat medium is circulated, wherein the heat medium is water and the heat medium pipes are copper pipes, the compressor and the heat source side heat exchanger being housed in an outdoor unit , the expansion device , the heat exchanger related to heat medium , and the pump being housed in a relay unit , each of the plurality of use side heat exchangers being housed in an indoor unit , the heat exchanger related to heat medium exchanging heat between the heat-source-side refrigerant and the heat medium, the refrigerant pipes are configured such that the inner cross-sectional area of each refrigerant pipe , which connects between the outdoor unit and the relay unit and through which a high-pressure refrigerant flows, is smaller than the inner cross-sectional area of the refrigerant pipe (4 ), which connects the outdoor unit and the relay unit and through which a low-pressure refrigerant flows, and the heat medium pipes, which connect the relay unit and the indoor units , are configured such that each inner cross-sectional area of the heat medium pipes per capacity unit is larger than the inner cross-sectional area per capacity unit of the refrigerant pipes . an outdoor unit ; an indoor unit ; and a relay unit ; a compressor ; a heat source side heat exchanger ; an expansion device ; a heat exchanger related to heat medium ; a pump; and a plurality of use side heat=exchangers , the compressor , the heat source side heat exchanger , the expansion device , and the heat exchanger related to heat medium being connected with two refrigerant pipes to form a refrigerant circuit in which a heat-source-side refrigerant is circulated, the pump, the plurality of use side heat exchangers , and the heat exchanger related to heat medium being connected with heat medium pipes to form a heat medium circuit in which a heat medium is circulated, wherein the heat medium is water and the heat medium pipes are copper pipes, the compressor and the heat source side heat exchanger being housed in an outdoor unit , the expansion device , the heat exchanger related to heat medium , and the pump being housed in a relay unit , each of the plurality of use side heat exchangers being housed in an indoor unit , the heat exchanger related to heat medium exchanging heat between the heat-source-side refrigerant and the heat medium, the refrigerant pipes are configured such that the inner cross-sectional area of each refrigerant pipe , which connects between the outdoor unit and the relay unit and through which a high-pressure refrigerant flows, is smaller than the inner cross-sectional area of the refrigerant pipe (4 ), which connects the outdoor unit and the relay unit and through which a low-pressure refrigerant flows, and the heat medium pipes, which connect the relay unit and the indoor units , are configured such that each inner cross-sectional area of the heat medium pipes per capacity unit is larger than the inner cross-sectional area per capacity unit of the refrigerant pipes . <EOS>