To prevent adverse influence from being exerted on proportional pressure control as dither effect becomes weak in an area in which a fed current is small because of characteristics of an electromagnet driving the spool of a spool valve type electromagnetic proportional pressure control valve and an area in which the current is so large that magnetic saturation is caused when the spool is swung by applying a dither without causing any stationary friction.

An AC dither current is superposed in the current fed to the electromagnet 14 to swing the spool 9 to a small amplitude exerting no influence on control liquid pressure and the amplitude of the dither current is made larger in a control area, in which the ratio FSOF/I of the fed current I and the force FSOL that a solenoid generates is smaller than the other control areas, than in other control areas. The frequency of the dither current may be made low instead of increasing the amplitude.