To reuse an existing tube of a refrigerator using R22 for a refrigerator using HFC refrigerant.

In the existing refrigerator using R22, an indoor unit B and an existing tube 21b are retained, and the other parts are removed. After the tube 21b is simply cleaned, a refrigerant-refrigerant heat exchanger 2 and a refrigerant pump 23 are connected to the tube 21b to constitute a secondary side circuit 20. The exchanger 2 is connected to a primary circuit 10. R407C is charged in the circuit 10 and the circuit 20. A designed pressure of a primary side tube 11 is set to larger than that of a secondary side tube 21 provided for R22.