An ultralow binary refrigeration apparatus, which cools an enclosed space to an ultralow temperature of −80° C. or lower to be used for storing cells or microorganisms, for example, without posing worries in regard of Ozone Depletion Potential (ODP) and Global Warming Potential (GWP), and which is excellent in refrigerant stability, can dispense with an oil separator, and enables a reduction in the amount of refrigerant, or a reduction in the power output of the compressor. The object is achieved by a binary refrigeration apparatus configured by connecting a high temperature refrigeration cycle and a low temperature refrigeration cycle via a cascade condenser, in which the high temperature refrigeration cycle is filled with propane as a refrigerant and a refrigerator oil, while the low temperature refrigeration cycle is filled with a hydrocarbon having a boiling point of −80° C. or lower as a refrigerant, a refrigerator oil, and an oil return agent.