PURPOSE: For very economical operation, to optimize the composition of the furan mixed refrigerant of at least three components at the final refrigerant evaporation temperatures of -70W100°C as well as the discharge pressure of a compressor and the outlet side pressure of an expansion valve.

CONSTITUTION: In a furan mixed refrigerant cooler, mixture refrigerant gas contains at least three components, i.e. R-12, R-13 and R-14 furan refrigerants before the pressure thereof is raised; said R-12, R-13 and R-14 furan refrigerants at the final refrigerant evaporation temperatures of -70W100°C are set to 35W50v ol% 10W40v o1% respectively; and the discharge pressure of a refrigerant compressor 21 and the outlet side pressure of an expansion valve (third) 30 are set to 8W14 kg/cm2G and 0W1 kg/cm2G respectively; and pipings are connected as illustrated. In the drawing, numeral 22 denotes a preheater, 23 and 25 gas and liquid separators, 24 and 26 heat exchangers, and 28 and 29 first and second expansion valves respectively.