A cooling unit (31a, 31b, 32a, 32b) comprising: a cooling device (33, 33a, 33b) which includes a casing (34a, 34b), a heat exchanger pipe (42a, 42b) with a difference in elevation in an upper and lower direction disposed in the casing, and a drain pan (50a, 50b) disposed below the heat exchanger pipe; a bypass pipe (52a, 53b, 54a, 54b) connected between an inlet path and an outlet path of the heat exchanger pipe and to form a CO 2 circulation path including the heat exchanger pipe; a bypass valve (53a, 53b) disposed in the bypass pipe (52a, 52b) and configured to be opened at a time of defrosting; an on-off valve (53a, 53b, 54a, 54b) which is disposed in each of the inlet path and the outlet path of the heat exchanger pipe and which is configured to be closed at a time of defrosting so that the CO 2 circulation path becomes a closed circuit; a pressure adjusting valve (48a, 48b) for adjusting pressure of the CO 2 refrigerant circulating in the closed circuit at the time of defrosting; characterised in further comprising: a brine circuit in which brine as a first heating medium circulates and which includes a first lead path (63a, 63b) disposed adjacent to the heat exchanger pipe in the cooling device (33a, 33b)-and forming a first heat exchanger part for heating the CO 2 refrigerant circulating in the heat exchanger pipe, with the brine, in a lower area of the heat exchanger pipe, and a second lead path (64a, 64b) led to the drain pan; and a flow path switching unit (69a, 69b) which enables the first lead path and the second lead path to be connected in parallel or connected in series. a cooling device (33, 33a, 33b) which includes a casing (34a, 34b), a heat exchanger pipe (42a, 42b) with a difference in elevation in an upper and lower direction disposed in the casing, and a drain pan (50a, 50b) disposed below the heat exchanger pipe; a bypass pipe (52a, 53b, 54a, 54b) connected between an inlet path and an outlet path of the heat exchanger pipe and to form a CO 2 circulation path including the heat exchanger pipe; a bypass valve (53a, 53b) disposed in the bypass pipe (52a, 52b) and configured to be opened at a time of defrosting; an on-off valve (53a, 53b, 54a, 54b) which is disposed in each of the inlet path and the outlet path of the heat exchanger pipe and which is configured to be closed at a time of defrosting so that the CO 2 circulation path becomes a closed circuit; a pressure adjusting valve (48a, 48b) for adjusting pressure of the CO 2 refrigerant circulating in the closed circuit at the time of defrosting; characterised in further comprising: a brine circuit in which brine as a first heating medium circulates and which includes a first lead path (63a, 63b) disposed adjacent to the heat exchanger pipe in the cooling device (33a, 33b)-and forming a first heat exchanger part for heating the CO 2 refrigerant circulating in the heat exchanger pipe, with the brine, in a lower area of the heat exchanger pipe, and a second lead path (64a, 64b) led to the drain pan; and a flow path switching unit (69a, 69b) which enables the first lead path and the second lead path to be connected in parallel or connected in series. a brine circuit in which brine as a first heating medium circulates and which includes a first lead path (63a, 63b) disposed adjacent to the heat exchanger pipe in the cooling device (33a, 33b)-and forming a first heat exchanger part for heating the CO 2 refrigerant circulating in the heat exchanger pipe, with the brine, in a lower area of the heat exchanger pipe, and a second lead path (64a, 64b) led to the drain pan; and a flow path switching unit (69a, 69b) which enables the first lead path and the second lead path to be connected in parallel or connected in series.