This invention provides a single cascade multistage turbine that has a compact turbine structure having only a single cascade, and enables power to be extracted efficiently in the same way as a multistage turbine having multistage rotor vanes. The working fluid supplied through the supply duct 3 flows on one side of the single cascade of rotor vanes 2 to one part of the circumference direction of the rotor vanes 2, and after passing through the rotor vanes 2 it passes through the circulation ducts 4a and 4b and returns to the former side of the rotor vanes 2. The circulation ducts 4a and 4b consist of exhaust duct members 6a and 6b, return duct members 7a and 7b, and inflow duct members 8a and 8b. Using the circulation ducts 4a and 4b this circulation process is repeated at least once whereby the working fluid is again passed through the rotor vanes 2 at a slightly different position in the circumferential direction of the rotor vanes 2. Each time the working fluid is passed through the rotor vanes 2, the energy contained in the fluid can be efficiently extracted as torque of the rotor vanes 2.