Abstract: In a refrigeration system having a refrigerant circuit (20) that is constructed in such a way that a plurality of lines of refrigeration circuits (70, 80) having refrigeration heat exchangers (72, 84) respectively are connected to an outdoor circuit (30) provided with an outdoor heat exchanger (32) and a compression mechanism (31) and conducts a vapor compression type refrigeration cycle, at least one of the lines of refrigeration circuits (80) having an auxiliary compressor (85) connected in series to the refrigeration heat exchanger (84), in order to respond to a variety of patterns of defrosting operations without providing a defrosting mechanism other than the refrigerant circuit (20), there is provided a hot gas introduction passage (46, 89) for selectively introducing gas refrigerant discharged from the compression mechanism (31) of the outdoor circuit (30) into at least one of the plurality of refrigeration heat exchangers (72, 84) and a defrosting operation of conducting a refrigeration cycle by using