A working fluid composition for a refrigerating machine comprising: a refrigerating machine oil containing at least one lubricating base oil selected from a mineral oil and a synthetic oil, and at least one phosphorus compound selected from a mono(alkylphenyl) diphenyl phosphate having a C3-C5 alkyl group and a di(alkylphenyl) phenyl phosphate having two C3-C5 alkyl groups, wherein a content of the phosphorus compound is 0.01 to 5% by mass based on a total amount of the refrigerating machine oil; and a hydrocarbon refrigerant having 2 to 4 carbon atoms.