A working fluid filled in a refrigerant circulation system comprising a compressor, a condenser, an expansion mechanism, an evaporator, and an accumulator, the working fluid comprising: a refrigerant; and a refrigerating machine oil, wherein the refrigerant comprises a hydrocarbon refrigerant having 2 to 4 carbon atoms, the refrigerating machine oil has a mixed aniline point of 20 C. or higher and 50 C. or lower and a viscosity index of 110 or more, an amount of the refrigerant dissolved in the working fluid is 40% by mass or less under conditions of a temperature of 80 C. and an absolute pressure of 2.8 MPa, and the refrigerating machine oil has a kinematic viscosity at 10 C. of 200 mm.sup.2/s or more and 3000 mm.sup.2/s or less.

A working fluid filled in a refrigerant circulation system comprising a compressor, a condenser, an expansion mechanism, an evaporator, and an accumulator, the working fluid comprising: a refrigerant; and a refrigerating machine oil, wherein the refrigerant comprises a hydrocarbon refrigerant having 2 to 4 carbon atoms, the refrigerating machine oil has a mixed aniline point of 20 C. or higher and 50 C. or lower and a viscosity index of 110 or more, an amount of the refrigerant dissolved in the working fluid is 40% by mass or less under conditions of a temperature of 80 C. and an absolute pressure of 2.8 MPa, and the refrigerating machine oil has a kinematic viscosity at 10 C. of 200 mm.sup.2/s or more and 3000 mm.sup.2/s or less.