A turbocharger is of reduced size and easy to assemble. A rotor blade upstream flow channel space has a cylindrical portion that is gradually reduced in the downstream direction of exhaust gas flow corresponding with a reduction in volume of the scroll chamber. A back plate is formed so as to swell out toward the rotor blade gradually in the downstream direction corresponding with the reduction in volume of the scroll chamber. The back plate, at a termination end thereof, approaches the rotor blade and reaches a position adjacent to a leading edge of the rotor blade whereby the flow channel space disappears substantially at this termination end.