A rotary expander which produces power by the expansion of supplied high-pressure fluid, the rotary expander comprising: a plurality of rotary mechanism parts (70, 80), each of which includes: a cylinder (71, 81) whose both ends are blocked; a piston (75, 85) for forming a fluid chamber (72, 82) in the cylinder (71, 81); and a blade (76, 86) for dividing the fluid chamber (72, 82) into a high-pressure chamber (73, 83) on the high-pressure side and a low-pressure chamber (74, 84) on the low-pressure side; and a rotating shaft which engages with the piston (75, 85) of each of the plural rotary mechanism parts (70, 80); characterised in that the plural rotary mechanism parts (70, 80) have different displacement volumes from each other, and are connected in series in ascending order of the different displacement volumes; in regard to two mutually connected rotary mechanism parts among the plural rotary mechanism parts (70, 80) one of which is a front-stage side rotary mechanism part and the other of which is a rear-stage side rotary mechanism part, the low-pressure chamber of the front-stage side rotary mechanism and the high-pressure chamber of the rear-stage side rotary mechanism part come into fluid communication with each other, resulting in the formation of a single expansion chamber, and fluid expands while flowing from the low-pressure chamber of the front-stage side rotary mechanism part into the high-pressure chamber of the rear-stage side rotary mechanism part; and the rotary expander includes: an injection passageway through which a part of the high-pressure fluid is introduced into the expansion chamber in the process of expansion; and a distribution control mechanism provided in the injection passageway. a plurality of rotary mechanism parts (70, 80), each of which includes: a cylinder (71, 81) whose both ends are blocked; a piston (75, 85) for forming a fluid chamber (72, 82) in the cylinder (71, 81); and a blade (76, 86) for dividing the fluid chamber (72, 82) into a high-pressure chamber (73, 83) on the high-pressure side and a low-pressure chamber (74, 84) on the low-pressure side; and a rotating shaft which engages with the piston (75, 85) of each of the plural rotary mechanism parts (70, 80); characterised in that the plural rotary mechanism parts (70, 80) have different displacement volumes from each other, and are connected in series in ascending order of the different displacement volumes; in regard to two mutually connected rotary mechanism parts among the plural rotary mechanism parts (70, 80) one of which is a front-stage side rotary mechanism part and the other of which is a rear-stage side rotary mechanism part, the low-pressure chamber of the front-stage side rotary mechanism and the high-pressure chamber of the rear-stage side rotary mechanism part come into fluid communication with each other, resulting in the formation of a single expansion chamber, and fluid expands while flowing from the low-pressure chamber of the front-stage side rotary mechanism part into the high-pressure chamber of the rear-stage side rotary mechanism part; and the rotary expander includes: an injection passageway through which a part of the high-pressure fluid is introduced into the expansion chamber in the process of expansion; and a distribution control mechanism provided in the injection passageway.