PURPOSE: To increase the accuracy of control by correcting the goal of the control or position detecting means in accordance with the position at which an actuator reaches its end.

CONSTITUTION: A CPU 21 is caused to function as control and correcting means, and a hydraulic actuator 3 is forced to move maximally in both directions, the outputs of a LVDT 5 delivered thereby are memorized in a memory 22 as its maximum and minimum displacements, and also the aimed values of the control are corrected respectively based on the maximum and minimum displacement. The gain errors and null-biass of the LVDT 5, a demodulator, etc. are therefore corrected properly relative to each other, so that the accuracy of the control can be increased without causing a rise in its cost or an increase in its weight. Additionally, practical matching of the hydraulic actuator 3 and its control electronics can be carried out.