An the expander-compressor unit (100) includes a closed casing (1) in which an oil can be stored in its bottom part, a compression mechanism (2) disposed in an upper part of the closed casing (1), an expansion mechanism (3) disposed in a lower part of the closed casing (1), a shaft (5) for coupling the compression mechanism (2) and the an expansion mechanism (3) to each other, and an oil pump (6), disposed between the compression mechanism (2) and the expansion mechanism (3), for supplying the oil (26) filling a surrounding space of the expansion mechanism (3) to the compression mechanism (2).