A reaction pressure control mechanism of power steering apparatus includes a power cylinder, to which a pressure oil discharged from an oil pump is supplied through a control valve, and oil discharged from the oil pump is branched to be introduced into a hydraulic reaction chamber, which produces steering reaction. A pair of variable restrictions are formed in a supply and discharge passage to and from the hydraulic reaction chamber, and a single spool which is operated by a solenoid is effective to increase the opening of one of the variable restrictions while reducing the opening of the other restriction. A plunger is disposed in opposing relationship with a rod associated with the solenoid and on which the spool is fixedly mounted so that when the oil pressure in the reaction chamber exceeds a given value, such oil pressure is introduced to the backside of the plunger to cause the spool to be driven back, thus reducing the opening of the variable restriction associated with the supply passage.