A free cooling outdoor unit comprising: a refrigerant circuit where a compressor, a first heat exchanger, a throttle device, and a refrigerant flow passage of a first water heat exchanger are connected by pipes so as to cause refrigerant to circulate therein; a brine circuit where a brine pump, a second heat exchanger, and a brine flow passage of a second water heat exchanger are connected by pipes so as to cause brine to circulate therein; a water circuit where a water pump, a water flow passage of the second water heat exchanger, and a water flow passage of the first water heat exchanger are connected by pipes so as to cause water to circulate therein; a fan configured to send air to the second heat exchanger; a flow control valve configured to control a circulation amount of the brine in the brine circuit; a water temperature detection sensor configured to detect a water temperature in the water circuit; an outside air temperature sensor configured to detect an outside air temperature; and a controller; characterised in that the controller is configured to control, in a case where the outside air temperature is equal to or lower than a freezing temperature of the brine, the circulation amount of the brine based on the water temperature such that a brine temperature is prevented from reaching a temperature equal to or lower than the freezing temperature of the brine; wherein in a case where the free cooling outdoor unit is in an operation state, and the controller determines that the outside air temperature is equal to or lower than the freezing temperature of the brine, and a difference between the water temperature and a target water temperature set in advance is equal to or less than a first reference temperature difference, the controller is configured to stop the fan and to operate the brine pump at a maximum rotation speed, and to control an opening degree of the flow control valve such that the circulation amount of the brine becomes a minimum flow rate. a refrigerant circuit where a compressor, a first heat exchanger, a throttle device, and a refrigerant flow passage of a first water heat exchanger are connected by pipes so as to cause refrigerant to circulate therein; a brine circuit where a brine pump, a second heat exchanger, and a brine flow passage of a second water heat exchanger are connected by pipes so as to cause brine to circulate therein; a water circuit where a water pump, a water flow passage of the second water heat exchanger, and a water flow passage of the first water heat exchanger are connected by pipes so as to cause water to circulate therein; a fan configured to send air to the second heat exchanger; a flow control valve configured to control a circulation amount of the brine in the brine circuit; a water temperature detection sensor configured to detect a water temperature in the water circuit; an outside air temperature sensor configured to detect an outside air temperature; and a controller; characterised in that the controller is configured to control, in a case where the outside air temperature is equal to or lower than a freezing temperature of the brine, the circulation amount of the brine based on the water temperature such that a brine temperature is prevented from reaching a temperature equal to or lower than the freezing temperature of the brine; wherein in a case where the free cooling outdoor unit is in an operation state, and the controller determines that the outside air temperature is equal to or lower than the freezing temperature of the brine, and a difference between the water temperature and a target water temperature set in advance is equal to or less than a first reference temperature difference, the controller is configured to stop the fan and to operate the brine pump at a maximum rotation speed, and to control an opening degree of the flow control valve such that the circulation amount of the brine becomes a minimum flow rate.