An expander-compressor unit (10) includes a two-stage rotary expansion mechanism (3) having a first cylinder (41) and a second cylinder (42). In the expansion mechanism (3), a suction port (71) facing a working chamber on the upstream side in the first cylinder (41) and a discharge port facing a working chamber on the downstream side in the second cylinder (42) are formed. An intermediate plate (43) is provided between the first cylinder (41) and the second cylinder (42). In the intermediate plate (43), a communication passage (43a) for allowing communication between a working chamber on the downstream side in the first cylinder (41) and a working chamber on the upstream side in the second cylinder (42) is formed. The communication passage (43a) does not communicate with the working chamber in the first cylinder (41) during the suction process, and communicates with the downstream working chamber in the first cylinder (41) at or after the end of the suction process.