A working fluid composition for a refrigerating machine, comprising: a refrigerating machine oil comprising, as a base oil, a mixed ester of (A) a complex ester synthesized from: (i) a polyhydric alcohol comprising at least one selected from neopentyl glycol, trimethylolpropane and pentaerythritol; (ii) a dihydric alcohol having 2 to 10 carbon atoms except neopentyl glycol; (iii) at least one selected from polybasic acids having 6 to 12 carbon atoms; and (iv) at least one selected from monohydric alcohols having 4 to 18 carbon atoms, and (B) a polyol ester synthesized from: (i) at least one polyhydric alcohol selected from neopentyl glycol, trimethylolpropane, pentaerythritol and dipentaerythritol; and (ii) at least one selected from monocarboxylic fatty acids having 4 to 18 carbon atoms, in a mass ratio of (A) the complex ester/(B) the polyol ester of 5/95 to 95/5; and a refrigerant selected from carbon dioxide, 1,1,1,2-tetrafluoroethane, a mixture of difluoromethane and pentafluoroethane, a mixture of difluoromethane, pentafluoroethane and 1,1,1,2-tetrafluoroethane, and a mixture of pentafluoroethane, 1,1,1,2-tetrafluoroethane and 1,1,1-trifluoroethane. a refrigerating machine oil comprising, as a base oil, a mixed ester of (A) a complex ester synthesized from: (i) a polyhydric alcohol comprising at least one selected from neopentyl glycol, trimethylolpropane and pentaerythritol; (ii) a dihydric alcohol having 2 to 10 carbon atoms except neopentyl glycol; (iii) at least one selected from polybasic acids having 6 to 12 carbon atoms; and (iv) at least one selected from monohydric alcohols having 4 to 18 carbon atoms, and (B) a polyol ester synthesized from: (i) at least one polyhydric alcohol selected from neopentyl glycol, trimethylolpropane, pentaerythritol and dipentaerythritol; and (ii) at least one selected from monocarboxylic fatty acids having 4 to 18 carbon atoms, in a mass ratio of (A) the complex ester/(B) the polyol ester of 5/95 to 95/5; and (A) a complex ester synthesized from: (i) a polyhydric alcohol comprising at least one selected from neopentyl glycol, trimethylolpropane and pentaerythritol; (ii) a dihydric alcohol having 2 to 10 carbon atoms except neopentyl glycol; (iii) at least one selected from polybasic acids having 6 to 12 carbon atoms; and (iv) at least one selected from monohydric alcohols having 4 to 18 carbon atoms, and (i) a polyhydric alcohol comprising at least one selected from neopentyl glycol, trimethylolpropane and pentaerythritol; (ii) a dihydric alcohol having 2 to 10 carbon atoms except neopentyl glycol; (iii) at least one selected from polybasic acids having 6 to 12 carbon atoms; and (iv) at least one selected from monohydric alcohols having 4 to 18 carbon atoms, and (B) a polyol ester synthesized from: (i) at least one polyhydric alcohol selected from neopentyl glycol, trimethylolpropane, pentaerythritol and dipentaerythritol; and (ii) at least one selected from monocarboxylic fatty acids having 4 to 18 carbon atoms, (i) at least one polyhydric alcohol selected from neopentyl glycol, trimethylolpropane, pentaerythritol and dipentaerythritol; and (ii) at least one selected from monocarboxylic fatty acids having 4 to 18 carbon atoms, a refrigerant selected from carbon dioxide, 1,1,1,2-tetrafluoroethane, a mixture of difluoromethane and pentafluoroethane, a mixture of difluoromethane, pentafluoroethane and 1,1,1,2-tetrafluoroethane, and a mixture of pentafluoroethane, 1,1,1,2-tetrafluoroethane and 1,1,1-trifluoroethane. <EOS>