To provide a refrigerator, which is an ultra-low temperature refrigerator for storing cells and microbes by cooling inside of the refrigerator at ultra-low temperature, free from a concern about an ozone destruction coefficient and a global warming coefficient and including a function for an energy saving operation.

A refrigerator, which stores an object to be frozen by setting set temperature in the device at ultra-low temperature of equal to or lower than -80°C, comprises by connecting a high temperature side refrigeration cycle in which a cooling medium including propane and refrigeration oil are filled and a low temperature side refrigeration cycle in which saturated hydrocarbon whose boiling point is equal to or lower than -80°C, refrigeration oil and an oil return agent are filled, by a cascade condenser. The problem can be solved by the refrigerator including control means capable of performing an energy saving operation by raising the set temperature within a range in which the object to be frozen is not damaged when outside air temperature becomes equal to or higher than a predetermined temperature.