PURPOSE: To avoid resonance phenomena generated in an oil pressure generating device and a high pressure oil piping by interposing a filter for damping only relatively high frequency components having no effect on normal controllability in a steam regulating valve opening signal requiring line, and thereby removing comparatively high frequency noise components.

CONSTITUTION: A high pressure oil supplied from a high pressure pump passes through an oil piping 12 and inflows into a steam regulating valve 13 through a servo valve 9. A number of revolution signal 1, that is, a control target parameter of a steam turbine, and a load- or steam pressure signal 2 are inputted to a calculating device 3 to become an opening requiring signal 4. When the signal frequencies of the number of revolution signal 1 and the load- or steam pressure signal 2 are less than specified values, a filter 14 directly outputs these signals as they are, and when frequency components higher than the specified exist, these signals are judged to be noise, and damped to be inputted to the steam regulating valve servo valve 9 as a steam regulating valve opening requiring signal 4' through a servo-amplifier 8 after its deviation 7 is calculated by a computing element 6. The resonance phenomena can thus be prevented.