A new refrigeration system is constituted by replacing a first heat source unit of an old system using an old refrigerant with a second heat source unit (A) using a new refrigerant. An oil separation apparatus (9) in the unit (A) returns oil to the compressor (1). A trap (13) separates extraneous matter from the returning oil and traps it. The old refrigerant is replaced by the new refrigerant. <IMAGE>
