A fluid machine is served as an expansion device and a compression device. The fluid machine compresses gas in an operation chamber upon functioning as the compression device. The fluid machine expands the gas in the operation chamber upon functioning as the expansion device. The fluid machine includes a movable discharge valve served as a differential pressure regulating valve that discharges the gas from the operation chamber when the fluid machine functions as the compression device. The discharge valve is moved to a non-operation position where the discharge valve fails to function as the differential pressure regulating valve when the fluid machine functions as the expansion device.