To drive a spool of a servo valve continuously by branching off a desired value into two parts, adding a bias signal to each signal whose one side is inverted, and controlling a volt-ampere converter for an electromagnet after getting these signals passed through a subtractor for load current feedback and a comparator for pulse width modulation.

A desired value Vi with the polarity extending over positive and negative is branched off into two parts before being added with a constantly set bias signal Vb, and one side comes to a signal Via as it is, but the other comes to a signal Vib with a code turned over. These two signals Via and Vib, after the bias signal Vb is added thereto, is subjected to minus in feedback a load current and is turned into a superposing signal, and then they pass through two pulse width modulating compactors 57 and 57' and are turned to control voltage, through which they control the current of an electromagnet by way of two volt-ampere converters 58 and 58' for each electromagnet. Here, when the desired value Vi is zero, a current equivalent to the bias signal Vb flows into each electromagnet with a constant current characteristic.