A working fluid composition for a refrigerating machine, comprising: a refrigerant comprising a hydrofluoroethane represented by the following formula (A), difluoromethane and tetrafluoropropene; and a refrigerating machine oil comprising at least one selected from a polyvinyl ether as a base oil, wherein a carbon/oxygen molar ratio of the base oil is 2.5 or more and 5.8 or less, and wherein the polyvinyl ether has a carbon/oxygen molar ratio of 4.0 or more and 5.0 or less and a weight average molecular weight of 200 or more and 3000 or less, wherein Formula (A) is as follows: C 2 H n F 6-n (A) wherein n represents 1 or 2, wherein the polyvinyl ether is a polyvinyl ether having a structural unit represented by the following formula : wherein R 1 , R 2 and R 3 each represent a hydrogen atom, m is 0, R 5 represents an alkyl group having 1 to 5 carbon atoms, and wherein the polyvinyl ether is a copolymer including: a structural unit (1-1) represented by the above formula , wherein R 5 represents an ethyl group, and a structural unit (1-2) represented by the above formula , wherein R 5 represents an isobutyl group, and wherein a molar ratio of the structural unit (1-1) to the structural unit (1-2) is 70 : 30 to 90 : 10. a refrigerant comprising a hydrofluoroethane represented by the following formula (A), difluoromethane and tetrafluoropropene; and a refrigerating machine oil comprising at least one selected from a polyvinyl ether as a base oil, wherein a carbon/oxygen molar ratio of the base oil is 2.5 or more and 5.8 or less, and wherein the polyvinyl ether has a carbon/oxygen molar ratio of 4.0 or more and 5.0 or less and a weight average molecular weight of 200 or more and 3000 or less, wherein Formula (A) is as follows: C 2 H n F 6-n (A) wherein n represents 1 or 2, wherein the polyvinyl ether is a polyvinyl ether having a structural unit represented by the following formula : wherein R 1 , R 2 and R 3 each represent a hydrogen atom, m is 0, R 5 represents an alkyl group having 1 to 5 carbon atoms, and wherein the polyvinyl ether is a copolymer including: a structural unit (1-1) represented by the above formula , wherein R 5 represents an ethyl group, and a structural unit (1-2) represented by the above formula , wherein R 5 represents an isobutyl group, and wherein a molar ratio of the structural unit (1-1) to the structural unit (1-2) is 70 : 30 to 90 : 10. wherein R 1 , R 2 and R 3 each represent a hydrogen atom, m is 0, R 5 represents an alkyl group having 1 to 5 carbon atoms, and wherein the polyvinyl ether is a copolymer including: a structural unit (1-1) represented by the above formula , wherein R 5 represents an ethyl group, and a structural unit (1-2) represented by the above formula , wherein R 5 represents an isobutyl group, and wherein a molar ratio of the structural unit (1-1) to the structural unit (1-2) is 70 : 30 to 90 : 10. a structural unit (1-1) represented by the above formula , wherein R 5 represents an ethyl group, and a structural unit (1-2) represented by the above formula , wherein R 5 represents an isobutyl group, and wherein a molar ratio of the structural unit (1-1) to the structural unit (1-2) is 70 : 30 to 90 : 10. <EOS>