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 internal spaces of a compressor casing (24) and an expander casing (34) communicate with each other through a pressure-equalizing pipe (41). An oil flow regulation valve (52) is provided in an oil flow pipe (42) connecting the compressor casing(24) to 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 oil reservoir (27) in the compressor casing (24) is allowed to communicate with the oil reservoir (37) in the expander casing (34), and the refrigerating machine oil moves through the oil flow pipe (42).