A refrigerant circuit (11) in an air conditioner (10) includes a compressor (20) and an expander (30). In the compressor (20), refrigerant compressed by a compression mechanism (21) is discharged into the internal space of a compressor casing (24). In the compressor (20), refrigeration oil which has accumulated in the bottom of the compressor casing (24) is supplied to the compression mechanism (21). The refrigeration oil in the bottom of the compressor casing (24) is directly introduced into an expansion mechanism (31) of the expander (30) through an oil supply pipe (41).