To obtain a refrigerant compsn. with a very low ozone depletion potential by using an azeotropic mixture of trifluoromethane and hexafluoroethane.

This azeotropic mixture, though poor in compatibility with an oil, can give a refrigerant compsn. which, when up to 14% (based on the mixture) n-pentane or propane is added to the mixture, can return the oil to a compressor without the necessity for completely separating the oil with an oil separator. This compsn. is used as a refrigerant sealed into the low-temp.-side refrigerant circuit of a two-way refrigerator wherein the condensation of a refrigerant in the low-temp.-side refrigerant circuit is carried out with a refrigerant passing through a cascade condenser of the high-temp.-side refrigerant circuit. Thus, the oil return in an actual refrigerant circuit is improved and such a low temp. as about -83°C can be attained in a vaporizer without being accompanied by the danger of explosion, etc.