A refrigeration apparatus including: a cooling device (33a, 33b) which is disposed in a freezer (30a, 30b), and includes a casing, a heat exchanger pipe led into the casing, and a drain receiver unit disposed below the heat exchanger pipe; a refrigerating device (11A, 11D) configured to cool and liquefy CO 2 refrigerant; and a refrigerant circuit (12, 14) connected to the heat exchanger pipe (42a, 42b), for permitting the CO 2 refrigerant cooled and liquefied by the refrigerating device to circulate to the heat exchanger pipe, and a defrost system comprising: a defrost circuit (52a, 52b) which is branched from an inlet path and an outlet path of the heat exchanger pipe and forms a CO 2 circulation path together with the heat exchanger pipe; an on-off valve (54a, 54b) which is disposed in each of the inlet path and the outlet path of the heat exchanger pipe (42a, 42b) and is configured to be closed at a time of defrosting so that the CO 2 circulation path becomes a closed circuit; a pressure adjusting unit (45a, 45b) for adjusting a pressure of the CO 2 refrigerant circulating in the closed circuit at the time of defrosting; characterized by further comprising: a first heat exchanger unit for heating the CO 2 refrigerant circulating in the defrost circuit with a brine, which is disposed below the cooling device (33a, 33b) and to which the defrost circuit and a first brine circuit in which the brine as a first heating medium circulates, are led, wherein the defrost circuit and the first brine circuit are configured to be led to the drain receiver unit, and the first heat exchanger unit includes the defrost circuit which is configured to be led to the drain receiver unit and the first brine circuit which is configured to be led to the drain receiver unit, the defrost system is configured to heat the drain receiver unit and the CO 2 refrigerant in the defrost circuit with the brine circulating in the first brine circuit, and the CO 2 refrigerant is permitted to naturally circulate in the closed circuit at the time of defrosting by a thermosiphon effect. a cooling device (33a, 33b) which is disposed in a freezer (30a, 30b), and includes a casing, a heat exchanger pipe led into the casing, and a drain receiver unit disposed below the heat exchanger pipe; a refrigerating device (11A, 11D) configured to cool and liquefy CO 2 refrigerant; and a refrigerant circuit (12, 14) connected to the heat exchanger pipe (42a, 42b), for permitting the CO 2 refrigerant cooled and liquefied by the refrigerating device to circulate to the heat exchanger pipe, and a defrost circuit (52a, 52b) which is branched from an inlet path and an outlet path of the heat exchanger pipe and forms a CO 2 circulation path together with the heat exchanger pipe; an on-off valve (54a, 54b) which is disposed in each of the inlet path and the outlet path of the heat exchanger pipe (42a, 42b) and is configured to be closed at a time of defrosting so that the CO 2 circulation path becomes a closed circuit; a pressure adjusting unit (45a, 45b) for adjusting a pressure of the CO 2 refrigerant circulating in the closed circuit at the time of defrosting; a first heat exchanger unit for heating the CO 2 refrigerant circulating in the defrost circuit with a brine, which is disposed below the cooling device (33a, 33b) and to which the defrost circuit and a first brine circuit in which the brine as a first heating medium circulates, are led, wherein the defrost circuit and the first brine circuit are configured to be led to the drain receiver unit, and the first heat exchanger unit includes the defrost circuit which is configured to be led to the drain receiver unit and the first brine circuit which is configured to be led to the drain receiver unit,