A working fluid composition for a refrigerating machine, comprising: a refrigerating machine oil comprising an ester of a polyhydric alcohol and a fatty acid, a carbodiimide compound selected from the group consisting of dialkylcarbodiimides, diphenylcarbodiimide, and bis(alkylphenyl)carbodiimides, and a phosphorus compound selected from the group consisting of phosphoric esters, acidic phosphoric esters, thiophosphates, amine salts of acidic phosphoric esters, chlorinated phosphoric esters, and phosphite esters; and at least one refrigerant selected from 1,1,1,2-tetrafluoroethane and 2,3,3,3-tetrafluoropropene; wherein the polyhydric alcohol constituting the ester comprises pentaerythritol and dipentaerythritol, the fatty acid constituting the ester consists of at least one selected from fatty acids having 4 to 9 carbon atoms, and the carbodiimide compound and the phosphorus compound satisfy conditions represented by the following formula: 0.11 ≦ N C M C ⋅ W C N P M P ⋅ W P ≦ 4.70 wherein, N C represents a number of carbodiimide groups per molecule of the carbodiimide compound, M C represents a molecular weight of the carbodiimide compound, W C represents a content of the carbodiimide compound based on a total amount of the refrigerating machine oil, N P represents a number of phosphorus atoms per molecule of the phosphorus compound, M P represents a molecular weight of the phosphorus compound, and W P represents a content of the phosphorus compound based on a total amount of the refrigerating machine oil, wherein the content of the carbodiimido compound is 0.01% by mass or more and 10% by mass or less, based on the total amount of the refrigerating machine oil, and wherein the content of the phosphorus compound is 0.1% by mass or more and 2.4% by mass or less, based on the total amount of the refrigerating machine oil. a refrigerating machine oil comprising an ester of a polyhydric alcohol and a fatty acid, a carbodiimide compound selected from the group consisting of dialkylcarbodiimides, diphenylcarbodiimide, and bis(alkylphenyl)carbodiimides, and a phosphorus compound selected from the group consisting of phosphoric esters, acidic phosphoric esters, thiophosphates, amine salts of acidic phosphoric esters, chlorinated phosphoric esters, and phosphite esters; and at least one refrigerant selected from 1,1,1,2-tetrafluoroethane and 2,3,3,3-tetrafluoropropene; wherein the polyhydric alcohol constituting the ester comprises pentaerythritol and dipentaerythritol, the fatty acid constituting the ester consists of at least one selected from fatty acids having 4 to 9 carbon atoms, and the carbodiimide compound and the phosphorus compound satisfy conditions represented by the following formula: 0.11 ≦ N C M C ⋅ W C N P M P ⋅ W P ≦ 4.70 wherein, N C represents a number of carbodiimide groups per molecule of the carbodiimide compound, M C represents a molecular weight of the carbodiimide compound, W C represents a content of the carbodiimide compound based on a total amount of the refrigerating machine oil, N P represents a number of phosphorus atoms per molecule of the phosphorus compound, M P represents a molecular weight of the phosphorus compound, and W P represents a content of the phosphorus compound based on a total amount of the refrigerating machine oil, wherein the content of the carbodiimido compound is 0.01% by mass or more and 10% by mass or less, based on the total amount of the refrigerating machine oil, and wherein the content of the phosphorus compound is 0.1% by mass or more and 2.4% by mass or less, based on the total amount of the refrigerating machine oil.