A heat transport system comprising: a refrigerant circuit that seals therein a fluid containing HFC-32 and/or HFO refrigerant as a refrigerant, the refrigerant circuit including: a refrigerant booster boosting the refrigerant; an outdoor air heat exchanger exchanging heat between the refrigerant and outdoor air; a medium heat exchanger exchanging heat between the refrigerant and a heat transfer medium; and a refrigerant flow path switching device switching between a refrigerant radiation state and a refrigerant evaporation state, the refrigerant radiation state causing the outdoor air heat exchanger to function as a radiator of the refrigerant and causing the medium heat exchanger to function as an evaporator of the refrigerant, the refrigerant evaporation state causing the outdoor air heat exchanger to function as an evaporator of the refrigerant and causing the medium heat exchanger 25) to function as a radiator of the refrigerant, and a medium circuit that seals carbon dioxide therein as the heat transfer medium, the medium circuit including: a medium booster boosting the heat transfer medium; the medium heat exchanger; a first medium flow path switching device switching between a first medium radiation state which causes the medium heat exchanger to function as a radiator of the heat transfer medium, and a first medium evaporation state which causes the medium heat exchanger to function as an evaporator of the heat transfer medium; and a plurality of indoor air heat exchangers (52a, 52b, 52c) exchanging heat between the heat transfer medium and indoor air, wherein the indoor air heat exchangers (52a, 52b, 52c) are fin-and-tube heat exchangers. a refrigerant circuit that seals therein a fluid containing HFC-32 and/or HFO refrigerant as a refrigerant, the refrigerant circuit including: a refrigerant booster boosting the refrigerant; an outdoor air heat exchanger exchanging heat between the refrigerant and outdoor air; a medium heat exchanger exchanging heat between the refrigerant and a heat transfer medium; and a refrigerant flow path switching device switching between a refrigerant radiation state and a refrigerant evaporation state, the refrigerant radiation state causing the outdoor air heat exchanger to function as a radiator of the refrigerant and causing the medium heat exchanger to function as an evaporator of the refrigerant, the refrigerant evaporation state causing the outdoor air heat exchanger to function as an evaporator of the refrigerant and causing the medium heat exchanger 25) to function as a radiator of the refrigerant, and a medium circuit that seals carbon dioxide therein as the heat transfer medium, the medium circuit including: a medium booster boosting the heat transfer medium; the medium heat exchanger; a first medium flow path switching device switching between a first medium radiation state which causes the medium heat exchanger to function as a radiator of the heat transfer medium, and a first medium evaporation state which causes the medium heat exchanger to function as an evaporator of the heat transfer medium; and a plurality of indoor air heat exchangers (52a, 52b, 52c) exchanging heat between the heat transfer medium and indoor air, wherein the indoor air heat exchangers (52a, 52b, 52c) are fin-and-tube heat exchangers. a refrigerant booster boosting the refrigerant; an outdoor air heat exchanger exchanging heat between the refrigerant and outdoor air; a medium heat exchanger exchanging heat between the refrigerant and a heat transfer medium; and a refrigerant flow path switching device switching between a refrigerant radiation state and a refrigerant evaporation state, the refrigerant radiation state causing the outdoor air heat exchanger to function as a radiator of the refrigerant and causing the medium heat exchanger to function as an evaporator of the refrigerant, the refrigerant evaporation state causing the outdoor air heat exchanger to function as an evaporator of the refrigerant and causing the medium heat exchanger 25) to function as a radiator of the refrigerant, and a medium circuit that seals carbon dioxide therein as the heat transfer medium, the medium circuit including: a medium booster boosting the heat transfer medium; the medium heat exchanger; a first medium flow path switching device switching between a first medium radiation state which causes the medium heat exchanger to function as a radiator of the heat transfer medium, and a first medium evaporation state which causes the medium heat exchanger to function as an evaporator of the heat transfer medium; and a plurality of indoor air heat exchangers (52a, 52b, 52c) exchanging heat between the heat transfer medium and indoor air, wherein the indoor air heat exchangers (52a, 52b, 52c) are fin-and-tube heat exchangers. a medium booster boosting the heat transfer medium; the medium heat exchanger; a first medium flow path switching device switching between a first medium radiation state which causes the medium heat exchanger to function as a radiator of the heat transfer medium, and a first medium evaporation state which causes the medium heat exchanger to function as an evaporator of the heat transfer medium; and a plurality of indoor air heat exchangers (52a, 52b, 52c) exchanging heat between the heat transfer medium and indoor air, wherein the indoor air heat exchangers (52a, 52b, 52c) are fin-and-tube heat exchangers.