An air-conditioning apparatus comprising: 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 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, 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 outdoor unit being connected to the relay unit through two refrigerant pipes and the relay unit being connected to each indoor unit through two heat medium pipes, the heat exchanger related to heat medium being configured to exchange heat between the heat-source-side refrigerant and the heat medium, the heat medium pipes being configured such that each inner cross-sectional area of the heat medium pipes per capacity unit is larger than the inner cross-sectional area of the refrigerant pipes per capacity unit through which a low pressure refrigerant flows, the expansion device and the heat exchanger related to heat medium being provided in plural numbers, the use side heat exchanger being one of a plurality of use side heat exchangers arranged in parallel, the apparatus having a cooling and heating mixed operation mode in which a high-temperature high-pressure heat-source-side refrigerant discharged from the compressor flows into one of the heat exchangers related to heat medium to heat the heat medium and a low-temperature low-pressure heat-source-side refrigerant flows into another one of the heat exchangers related to heat medium to cool the heat medium such that each use side heat exchanger is allowed to perform a cooling operation or a heating operation, one of the expansion devices being disposed on an outlet side of the heat exchanger related to heat medium on the heating side in the cooling and heating mixed operation mode and another one of the expansion devices being disposed on an inlet side of the heat exchanger related to heat medium on the cooling side in the cooling and heating mixed operation mode, wherein the heat medium is water and the heat medium pipes are copper pipes. 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 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, 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 outdoor unit being connected to the relay unit through two refrigerant pipes and the relay unit being connected to each indoor unit through two heat medium pipes, the heat exchanger related to heat medium being configured to exchange heat between the heat-source-side refrigerant and the heat medium, the heat medium pipes being configured such that each inner cross-sectional area of the heat medium pipes per capacity unit is larger than the inner cross-sectional area of the refrigerant pipes per capacity unit through which a low pressure refrigerant flows, the expansion device and the heat exchanger related to heat medium being provided in plural numbers, the use side heat exchanger being one of a plurality of use side heat exchangers arranged in parallel, the apparatus having a cooling and heating mixed operation mode in which a high-temperature high-pressure heat-source-side refrigerant discharged from the compressor flows into one of the heat exchangers related to heat medium to heat the heat medium and a low-temperature low-pressure heat-source-side refrigerant flows into another one of the heat exchangers related to heat medium to cool the heat medium such that each use side heat exchanger is allowed to perform a cooling operation or a heating operation, one of the expansion devices being disposed on an outlet side of the heat exchanger related to heat medium on the heating side in the cooling and heating mixed operation mode and another one of the expansion devices being disposed on an inlet side of the heat exchanger related to heat medium on the cooling side in the cooling and heating mixed operation mode, wherein the heat medium is water and the heat medium pipes are copper pipes.