A compressor (20) and an expander (30) are provided in the refrigerant circuit (11) of an air conditioner (10). In the compressor (20), a refrigerating machine oil is fed from an oil reservoir (27) into a compression mechanism (21). In the expander (30), a refrigerating machine oil is fed from an oil reservoir (37) into an expansion mechanism (31). The interior of the compressor casing (24) is in a refrigerating cycle with high pressure and the interior of the expander casing (34) is in a refrigerating cycle with low pressure. An oil flow regulation valve (52) is provided in an oil flow pipe (42) allowing the compressor casing (24) to communicate with the expander casing (34). The oil flow regulation valve (52) is operated according to the output signals from an oil level sensor (51). When the oil flow regulation valve (52) is opened, the refrigerating machine oil flows through the oil flow pipe (42) from the oil reservoir (27) in the compressor casing (24) into the oil reservoir (37) in the expander casing (34).