A positive displacement expander having within a casing an expansion mechanism for generating power by the expansion of fluid in an expansion chamber, the casing further containing therein pressure snubbing means for inhibiting at least either variation in the pressure of fluid which is drawn into the expansion chamber or variation in the pressure of fluid which is discharged out of the expansion chamber, characterized in that the expansion mechanism is provided with a suction passageway for introducing fluid into the expansion chamber and a discharge passageway for discharging fluid after expansion from the expansion chamber, the pressure snubbing means is provided with a pressure snubbing chamber which is so configured as to perform, in response to fluid pressure variation, suction of fluid from and discharge of fluid into either the suction passageway or the discharge passageway, the pressure snubbing chamber of the pressure snubbing means is formed within a forming member (61, 62) of the expansion chamber, a fluid compression mechanism is provided within the casing and an internal space of the casing is filled up with fluid compressed by the compression mechanism, and the pressure snubbing chamber comprises (i) a fluid inflow/outflow chamber in fluid communication with either the suction passageway or the discharge passageway, (ii) a back pressure chamber in fluid communication with the internal space of the casing, and (iii) a partitioning member which separates the inflow/outflow chamber and the back pressure chamber and which is displaceably configured such that the volume of the inflow/outflow chamber varies in response to fluid pressure variation. the casing further containing therein pressure snubbing means for inhibiting at least either variation in the pressure of fluid which is drawn into the expansion chamber or variation in the pressure of fluid which is discharged out of the expansion chamber, characterized in that the expansion mechanism is provided with a suction passageway for introducing fluid into the expansion chamber and a discharge passageway for discharging fluid after expansion from the expansion chamber, the pressure snubbing means is provided with a pressure snubbing chamber which is so configured as to perform, in response to fluid pressure variation, suction of fluid from and discharge of fluid into either the suction passageway or the discharge passageway, the pressure snubbing chamber of the pressure snubbing means is formed within a forming member (61, 62) of the expansion chamber, a fluid compression mechanism is provided within the casing and an internal space of the casing is filled up with fluid compressed by the compression mechanism, and the pressure snubbing chamber comprises (i) a fluid inflow/outflow chamber in fluid communication with either the suction passageway or the discharge passageway, (ii) a back pressure chamber in fluid communication with the internal space of the casing, and (iii) a partitioning member which separates the inflow/outflow chamber and the back pressure chamber and which is displaceably configured such that the volume of the inflow/outflow chamber varies in response to fluid pressure variation.