To provide a method of operating a unit cooler capable of performing optimal cooling operation according to a cooling load by selecting cooling capacity in accordance with a state of the interior cooling load.

In the method of operating the unit cooler 1 for a cooling storage including a cooling coil 5 arranged in an interior upper part of the cooling storage 3 and a fan 8 blowing out interior air sucked from a suction port 6 and cooled by the cooling coil 5 from a blowout port 7, a plurality of cooling coils 5a, 5b with different cooling capacity are provided as the cooling coil 5. One or a plurality of cooling coils are selected from the cooling coils 5a, 5b in accordance with the interior cooling load, and a refrigerant is introduced.