Hydraulic power steering apparatus

The control valve (30) of the hydraulic power steering apparatus controls hydraulic pressure of the oil supplied to the hydraulic actuator (20) for generating steering assist power, in accordance with the amount of elastic relative rotation between the input and output shafts (2, 3). The restricting device (50) restricting the relative rotation between the input and output shafts (2, 3) in accordance with a driving condition of the vehicle adjusts the hydraulic pressure for pressing one of the input and output shaft (2, 3) via the plunger (52) inserted into the guide bore (51) formed in the other of the input and output shaft (2, 3), in accordance with the driving condition of the vehicle by means of the hydraulic pressure adjustment mechanism (60). The plunger (52) is guided by the inner circumference of the guide bore (51) to the direction in which the plunger (52) approaches to and goes away from the other of the input and output shaft (2, 3) when the input and output shafts (2, 3) relatively rotate. The friction reducing element (53, 70, 80) is interposed between the inner circumference of the guide bore (51) and the outer circumference of the plunger (52).