A refrigerating machine oil comprising an ester of a polyhydric alcohol and a fatty acid, wherein the molar ratio of C 4 -C 6 fatty acid and C 7 -C 9 branched fatty acid in the fatty acid is between 30:70 and 70:30, the C 4 -C 6 fatty acid includes 2-methylpropanoic acid, and the ratio of the total C 4 -C 6 fatty acid and C 7 -C 9 branched fatty acid in the total fatty acids composing the ester is at least 20 mol% wherein the polyhydric alcohol is pentaerythritol and the 40°C kinematic viscosity of the refrigerating machine oil is 20 to 80 mm 2 /s; and wherein the C 4 -C 6 fatty acid is selected from the group consisting of butanoic acid, 2-methylpropanoic acid, pentanoic acid, 2-methylbutanoic acid, 3-methylbutanoic acid, 2,2-dimethylpropanoic acid, 2-methylpentanoic acid, 3-methylpentanoic acid, 4-methylpentanoic acid, 2,2-dimethylbutanoic acid, 2,3-dimethylbutanoic acid, 3,3-dimethylbutanoic acid and hexanoic acid; and wherein the branched C 7 -C 9 fatty acid is selected from the group consisting of octanoic acid, 2-ethylhexanoic acid, 3-ethylhexanoic acid, 3,5-dimethylhexanoic acid, 2,4-dimethylhexanoic acid, 3,4-dimethylhexanoic acid, 4,5-dimethylhexanoic acid, 2,2-dimethylhexanoic acid, 2-methylheptanoic acid, 3-methylheptanoic acid, 4-methylheptanoic acid, 5-methylheptanoic acid, 6-methylheptanoic acid, 2-propylpentanoic acid, nonanoic acid, 2,2-dimethylheptanoic acid, 2-methyloctanoic acid, 2-ethylheptanoic acid, 3-methyloctanoic acid, 3,5,5-trimethylhexanoic acid, 2-ethyl-2,3,3-trimethylbutyric acid, 2,2,4,4-tetramethylpentanoic acid, 2,2,3,3-tetramethylpentanoic acid, 2,2,3,4-tetramethylpentanoic acid and 2,2-diisopropylpropanoic acid; and wherein the non C 4 -C 6 fatty acids and C 7 -C 9 branched fatty acids are selected from the group consisting of acetic acid, propionic acid; C 7 -C 9 straight-chain fatty acids and C 10 -C 20 fatty acids. the C 4 -C 6 fatty acid includes 2-methylpropanoic acid, and the ratio of the total C 4 -C 6 fatty acid and C 7 -C 9 branched fatty acid in the total fatty acids composing the ester is at least 20 mol% wherein the polyhydric alcohol is pentaerythritol and the 40°C kinematic viscosity of the refrigerating machine oil is 20 to 80 mm 2 /s; and wherein the C 4 -C 6 fatty acid is selected from the group consisting of butanoic acid, 2-methylpropanoic acid, pentanoic acid, 2-methylbutanoic acid, 3-methylbutanoic acid, 2,2-dimethylpropanoic acid, 2-methylpentanoic acid, 3-methylpentanoic acid, 4-methylpentanoic acid, 2,2-dimethylbutanoic acid, 2,3-dimethylbutanoic acid, 3,3-dimethylbutanoic acid and hexanoic acid; and wherein the branched C 7 -C 9 fatty acid is selected from the group consisting of octanoic acid, 2-ethylhexanoic acid, 3-ethylhexanoic acid, 3,5-dimethylhexanoic acid, 2,4-dimethylhexanoic acid, 3,4-dimethylhexanoic acid, 4,5-dimethylhexanoic acid, 2,2-dimethylhexanoic acid, 2-methylheptanoic acid, 3-methylheptanoic acid, 4-methylheptanoic acid, 5-methylheptanoic acid, 6-methylheptanoic acid, 2-propylpentanoic acid, nonanoic acid, 2,2-dimethylheptanoic acid, 2-methyloctanoic acid, 2-ethylheptanoic acid, 3-methyloctanoic acid, 3,5,5-trimethylhexanoic acid, 2-ethyl-2,3,3-trimethylbutyric acid, 2,2,4,4-tetramethylpentanoic acid, 2,2,3,3-tetramethylpentanoic acid, 2,2,3,4-tetramethylpentanoic acid and 2,2-diisopropylpropanoic acid; and wherein the non C 4 -C 6 fatty acids and C 7 -C 9 branched fatty acids are selected from the group consisting of acetic acid, propionic acid; C 7 -C 9 straight-chain fatty acids and C 10 -C 20 fatty acids. <EOS>