A control device for a chilling unit including an air heat exchanger configured to send out refrigerant heat-exchanged with the outside air, a water heat exchanger configured to cool water by exchanging heat between the refrigerant sent out from the air heat exchanger and the water, a compressor configured to compress the refrigerant and to send out the compressed refrigerant, a first refrigerant pipe configured to send out the refrigerant sent out from the compressor to the air heat exchanger, and a second refrigerant pipe provided between the compressor and the water heat exchanger, the control device comprising: a first pressure sensor configured to detect a refrigerant pressure in the second refrigerant pipe; a second pressure sensor configured to detect a refrigerant pressure in the first refrigerant pipe; and a control unit configured to control an operation of the chilling unit, wherein the control in which the control unit is configured to perform includes: controlling a differential pressure obtained by subtracting a refrigerant pressure detected by the first pressure sensor from a refrigerant pressure detected by the second pressure sensor to be equal to or greater than a predetermined differential pressure at which the refrigerant circulates and the water does not freeze when a cooling cycle to cool the water using the water heat exchanger is operated; causing the chilling unit to operate a reverse cycle of the cooling cycle before operation of the cooling cycle is started in the chilling unit when the outside air temperature is an outside air temperature that makes the temperature of the water equal to or lower than the freezing point of the water; and causing the operation of the reverse cycle to be switched to operation of the cooling cycle at a timing at which a differential pressure of the refrigerant obtained by subtracting a pressure detected by the first pressure sensor from a pressure detected by the second pressure sensor becomes within a predetermined range at which the refrigerant is capable of being flown with the same circulation flow rate of the refrigerant when the chilling unit reaches an equilibrium state; wherein the control unit is configured to switch from the operation of the reverse cycle of the cooling cycle to the operation of the cooling cycle at different timings for each of the plurality of sets, when the chilling unit includes a plurality of sets that each include an air heat exchanger, a water heat exchanger, and a compressor. a first pressure sensor configured to detect a refrigerant pressure in the second refrigerant pipe; a second pressure sensor configured to detect a refrigerant pressure in the first refrigerant pipe; and a control unit configured to control an operation of the chilling unit, wherein the control in which the control unit is configured to perform includes: controlling a differential pressure obtained by subtracting a refrigerant pressure detected by the first pressure sensor from a refrigerant pressure detected by the second pressure sensor to be equal to or greater than a predetermined differential pressure at which the refrigerant circulates and the water does not freeze when a cooling cycle to cool the water using the water heat exchanger is operated; causing the chilling unit to operate a reverse cycle of the cooling cycle before operation of the cooling cycle is started in the chilling unit when the outside air temperature is an outside air temperature that makes the temperature of the water equal to or lower than the freezing point of the water; and causing the operation of the reverse cycle to be switched to operation of the cooling cycle at a timing at which a differential pressure of the refrigerant obtained by subtracting a pressure detected by the first pressure sensor from a pressure detected by the second pressure sensor becomes within a predetermined range at which the refrigerant is capable of being flown with the same circulation flow rate of the refrigerant when the chilling unit reaches an equilibrium state; controlling a differential pressure obtained by subtracting a refrigerant pressure detected by the first pressure sensor from a refrigerant pressure detected by the second pressure sensor to be equal to or greater than a predetermined differential pressure at which the refrigerant circulates and the water does not freeze when a cooling cycle to cool the water using the water heat exchanger is operated; causing the chilling unit to operate a reverse cycle of the cooling cycle before operation of the cooling cycle is started in the chilling unit when the outside air temperature is an outside air temperature that makes the temperature of the water equal to or lower than the freezing point of the water; and causing the operation of the reverse cycle to be switched to operation of the cooling cycle at a timing at which a differential pressure of the refrigerant obtained by subtracting a pressure detected by the first pressure sensor from a pressure detected by the second pressure sensor becomes within a predetermined range at which the refrigerant is capable of being flown with the same circulation flow rate of the refrigerant when the chilling unit reaches an equilibrium state; wherein the control unit is configured to switch from the operation of the reverse cycle of the cooling cycle to the operation of the cooling cycle at different timings for each of the plurality of sets, when the chilling unit includes a plurality of sets that each include an air heat exchanger, a water heat exchanger, and a compressor.