PURPOSE: To make it possible, in operating plural refrigerant-refining apparatuses installed in a refrigeration system through alternate change, to conduct automatic change of the plural refrigerant-refining apparatus always with time to spare, by constituting the system in such a way that the purity of intermediate refrigerant obtained by the refining apparatus can be detected.

CONSTITUTION: When the detection of the purity of circulating refrigerant such as He etc., which was branched through an automatic valve 15 attached to the outlet of a adsorbing cylinder 13 in a refining apparatus 6 equipped in a refrigeration system, shows that the purity is an intermediate value on the way to the allowable limit of purity, or to the decrease in dew point, supply of the refrigerant to a refining apparatus 6 is stopped according to the signal given by a detecting means 8, and supply of the refrigerant to a refining apparatus 7 is started in order to conduct refining. When an intermediate purity of the refrigerant is detected during operation of the refining apparatus 7, the apparatus 7 is stopped from operating, and the apparatus 6 is started, then regeneration is carried out in the apparatus 7 with an automatic valve 18 opened. Because the apparatuses 6, 7 are changed alternately always with time to spare, insufficiently refined refrigerant is scarcely charged to the liquefying refrigerator 1, consequently the refrigeration system is operated without interruption for a long time. In Figure, 2 is a container of liq. He, 3 is a cryostat, 4 is a recovered-gas bag, and 5 is a compressor.