PURPOSE: To provide a controller with high reliability and efficiency that needs neither ball nuts nor ball screws by forming a rod which is displaced in a linear direction, and a water rate adjusting device which is located on the inlet side of a waterwheel and opens/closes along with displacement of the rod in the axial direction.

CONSTITUTION: This controller uses a reaction type waterwheel as its waterwheel and a linear motor 14 as its direct-operated electromagnetic actuator. A direct-operated rod 25 of the linear motor 14 is displaced in the axial direction by changing electromagnetic force, driving a water rate adjusting device which opens/closes a guide vane 1 of the reaction type waterwheel formed on the inlet side of the waterwheel. The guide vane 1 rotates with a change in position in the axial direction of the direct-operated rod 25 of the linear motor 14, thereby opening or closing the water rate adjusting device. It is thus possible to eliminate a mechanical conversion device with a screw such as a ball screw, and the possibility of galling, seizure or twisting and improve reliability of operation of the controller largely.