PURPOSE: To enable a suitable correction of driving on-time by considering mechanical dispersion of a proportional flow-control valve and change of the oil temperature.

CONSTITUTION: A specified section for measurement of velocity is set and a proportional flow-control valve is driven for a prescribed time according to a prescribed on-time to measure the velocity. The driving on-time is corrected corresponding to the difference from the prescribed speed while controlling the above-mentioned driving on-time so as to make a correction only when the controlled set-on time is beyond a prescribed value.