To provide a cooling system having high safety by inhibiting pressure rise of a secondary refrigerant circuit during the stop of an operation, being stably operated without generating cavitation in a liquid pump even when sudden change of load occurs during the operation, and reducing device costs and running costs.

This natural refrigerant cooling system comprises a primary refrigerant circuit 1 and a secondary refrigerant circuit 2, and further comprises a vapor phase cooling coil 18 cooling a vapor phase of a refrigerant in a receiver 7 of the secondary refrigerant circuit, and a liquid phase cooling coil 17 cooling a liquid phase. In the stop of operation when the cooling to a load-side cooler 9 is stopped, a cooling medium is supplied to the gas phase cooling coil 18 according to a pressure in the receiver 7, and in a normal operation when the cooling to the load-side cooler is performed, the cooling medium is supplied to the liquid phase cooling coil 17 to supercool a liquid refrigerant.