A steering reaction force generating mechanism for generating a reaction force acting on an input shaft is provided within a cylindrical space of a housing between a lower end portion of the input shaft and an upper end portion of a pinion shaft (output shaft). The steering reaction force generating mechanism includes an input shaft accommodating section for accommodating the lower end of the input shaft together with its two control arms which protrude symmetrically and radially therefrom. A pair of reaction force chambers are provided at both sides of the input shaft accommodating section in parallel with the two control arms. Two pairs of through bores are formed respectively between one of the reaction force chambers and the input shaft accommodating section, and between the other of the reaction force chambers and the input shaft accommodating section. Two pairs of plungers are respectively disposed in the through bores so that each plunger is slidable in its axial direction toward and away from the control arm of the input shaft. The movement of each plunger is restricted by a pin inserted into a further through bore formed in the plunger extending in a direction orthogonal to the axis of the plunger. The inner peripheral surface of the through bore and the outer peripheral surface of the plunger can be ground easily and accurately to insure smooth movement of the plunger.