PURPOSE: To control rapid fluctuation of a moisture separator drain tank water level by stepwisely and forcedly opening a water level regulating valve of an emergency line if rate of change of high pressure turbine outlet pressure by turbine load change exceeds prescribed one.

CONSTITUTION: A pressure detector 15 in which the outlet pressure of a high pressure turbine 2 is detected, a differential operator 16 in which the rate of the change of the pressure is operated and a function generator 17 in which regulator output for the rate of the change of the pressure is prescribed are provided on an emergency line B. When turbine load is decreased, at the same time the outlet pressure of the turbine 2 is decreased, but at the time of large pressure decrease rate the drain in a moisture separator drain tank 6 is gas- liquid two-phase flow, the volume is increased and the water level is heightened. So the rate of the change of the pressure is detected, an emergency regulating valve 12 is opened on the basis of the regulator output 19 prescribed by the generator 17 precedently, and the drain is discharged to a condenser 14. Therefore, stable water level can be controlled while holding the increase of the water level to the minimum.