A working fluid composition for a refrigerating machine, comprising: a refrigerating machine oil comprising, as a base oil, a mixed ester of a complex ester and a polyol ester, wherein the complex ester is obtainable by synthesis of at least one polyhydric alcohol selected from neopentyl glycol, trimethylolpropane and pentaerythritol, a polybasic acid having 6 to 12 carbon atoms, and a monohydric alcohol having 4 to 18 carbon atoms or a monocarboxylic acid having 4 to 18 carbon atoms, the polybasic acid being used in an amount of 0.4 to 4 mol based on 1 mol of the polyhydric alcohol, wherein the polyol ester is obtainable by synthesis of at least one polyhydric alcohol selected from neopentyl glycol, trimethylolpropane, pentaerythritol and dipentaerythritol, and a monocarboxylic acid having 4 to 18 carbon atoms, and wherein a mass ratio of the complex ester/ the polyol ester is 5/95 to 95/5, and a total content of the complex ester and the polyol ester is 80% by mass or more of the refrigerating machine oil; and difluoromethane used as a refrigerant, the content of difluoromethane in the refrigerant being 60 to 100% by mass; wherein an amount of the refrigerating machine oil is 2 to 400 parts by mass based on 100 parts by mass of the refrigerant; wherein the working fluid composition has a refrigerant dissolved viscosity of 5.5 mm 2 /s or more, at a temperature of 80°C and an absolute pressure of 2.1 MPa; measured in a 200 ml pressure resistant vessel filled with 100 g of the refrigerating machine oil by adding the refrigerant to an absolute pressure of 2.1 MPa; and wherein the complex ester has a kinematic viscosity of 40 to 400 mm 2 /s at 40°C. a refrigerating machine oil comprising, as a base oil, a mixed ester of a complex ester and a polyol ester, wherein the complex ester is obtainable by synthesis of at least one polyhydric alcohol selected from neopentyl glycol, trimethylolpropane and pentaerythritol, a polybasic acid having 6 to 12 carbon atoms, and a monohydric alcohol having 4 to 18 carbon atoms or a monocarboxylic acid having 4 to 18 carbon atoms, the polybasic acid being used in an amount of 0.4 to 4 mol based on 1 mol of the polyhydric alcohol, wherein the polyol ester is obtainable by synthesis of at least one polyhydric alcohol selected from neopentyl glycol, trimethylolpropane, pentaerythritol and dipentaerythritol, and a monocarboxylic acid having 4 to 18 carbon atoms, and wherein a mass ratio of the complex ester/ the polyol ester is 5/95 to 95/5, and a total content of the complex ester and the polyol ester is 80% by mass or more of the refrigerating machine oil; and difluoromethane used as a refrigerant, the content of difluoromethane in the refrigerant being 60 to 100% by mass; wherein an amount of the refrigerating machine oil is 2 to 400 parts by mass based on 100 parts by mass of the refrigerant; wherein the working fluid composition has a refrigerant dissolved viscosity of 5.5 mm 2 /s or more, at a temperature of 80°C and an absolute pressure of 2.1 MPa; measured in a 200 ml pressure resistant vessel filled with 100 g of the refrigerating machine oil by adding the refrigerant to an absolute pressure of 2.1 MPa; and wherein the complex ester has a kinematic viscosity of 40 to 400 mm 2 /s at 40°C.