A positive displacement expander provided with an expansion mechanism for generating power using fluid energy occurring when a high-pressure fluid supplied to expansion chambers (81a, 82a), which are divided by an orbiting scroll or a rocking piston, is reduced in pressure by expansion. The positive displacement expander is also provided with a communication path (71) for connecting each of the expansion chambers (81a, 82a) and the discharge side of the expander. An opening and closing device (72) is provided in the communication path (71). When the supply of the high-pressure fluid is stopped, the opening and closing device (72) stops the orbiting scroll or the rocking piston at a predetermined position by opening during a time period until high pressure and low pressure between each of the expansion chambers (81a, 82a) and the discharge side of the expander are equalized. As a result, the expander (8) can have sufficient drive power at the time of restarting.