A refrigeration cycle system includes a compressor 1, a condenser 2, an expansion device 3 and an evaporator 5 connected in series with each other, and charged with a heteroazeotropic refrigerant mixture comprising R12 and R13. A fractionating vessel 6 and an electromagnetic valve 7 are provided in predetermined portions of the system for separating the R12 and R13 refrigerant components during an ordinary freezing mode and for mixing them during a rapid freezing mode.