The base oil for the refrigerating machine oil used with a carbon dioxide refrigerant of the invention is characterized by comprising a complete ester of a fatty acid in which the proportion of C14-C22 branched fatty acid is 40-100% by mole and a polyhydric alcohol. The refrigerating machine oil used with a carbon dioxide refrigerant according to the invention is characterized by comprising the base oil for the refrigerating machine oil used with a carbon dioxide refrigerant according to the invention. The base oil for the refrigerating machine oil used with a carbon dioxide refrigerant and the refrigerating machine oil used with a carbon dioxide refrigerant according to the invention, when used together with a carbon dioxide refrigerant, exhibit excellent stability and electrical insulating properties, and have suitable compatibility with refrigerants while allowing adequate lubricity to be exhibited without increasing the viscosity of the base oil.