For example, Japanese Patent Application Kokai No. 1-259093 discloses ???a refrigerating machine oil for a flon compressor??? which comprises as base oil a propylene glycol monoether represented by the general formula: wherein R is an alkyl group having 1 to 8 carbon atoms, and n is an integer of 4 to 19; Japanese Patent Application Kokai No. 1-259094 discloses a diether type compound obtained by