A scroll expander has an expansion mechanism (5) constructed from a swing scroll (52) and a first fixed scroll (51) and recovering power by expanding refrigerant, a sub compression mechanism (6) constructed from a swing scroll (62) and a second fixed scroll (61), driven by the power recovered by the expansion mechanism (5), and compressing the refrigerant, and a seal ring (23) fitted to at least either the outer periphery of the sub compression mechanism (6) or the outer periphery of the expansion mechanism (5). An oil passage (17) opened to an upper space (70) inside a closed container (10) is formed in the expander so that the pressure in the upper space (70) and a lowers pace (72) is the pressure after compression is made by the sub compression mechanism (6). Oil piping (80) communicating with a main compressor (11) is installed in the lower space (72). The scroll expander can recover power with a minimum loss by a simple structure, operates efficiently in a wide range of operating conditions, and is manufacturable at low cost.