PURPOSE: To maintain the merits of a simple control of a spiral means while avoiding the demerits of the spiral means, and maintain a conventional structure with a control blade wheel operating according to the impulse principle.

CONSTITUTION: An admission casing has a symmetrical tours member 4. A plurality of steam supply ports 1, 2 open in tangential direction toward the admission casing. The cross-section area of the tangential steam supply ports 1, 2 is designed for different mass flows. An exit of the admission casing has an annular cross section member 5. The annular cross section member has a structure such that the work medium flowing into a blade arrangement includes tangential components irrelative to operation load. The tangential component is within the order value of the circumferential velocity of a first stage blade sector pressed by the work medium.