PURPOSE: To provide a method and a device for incrementally controlling a flow of a first stage of 9 turbine without adding or throtting a flow control valve.

CONSTITUTION: In an atomic power steam turbine 18, deterioration of a heat consumption ratio is generated due to excessive flow margine. For minimizing this, a plug member 48 selectively disconnects a flow passage for steam passing a first stage nozzle ring 20. The plug member 48 is selectively inserted before the flow passage between adjacent nozzle blades 30 of the nozzle ring 20 by detection of a timing when a flow of steam passing a control valve to supply steam to the steam turbine 18 exceeds a desired flow value to the steam turbine 18 by a monitoring device. The plug member 48 effectively reduces the flow passing the nozzle ring 20 without throttling steam by the control valve to prevent deterioration of efficiency.