PURPOSE: To reduce generation of cavitation so as to suppress a noise of fluid by providing constitution such that viscosity of electroviscous fluid between a valve shaft and a valve body is changed in accordance with a value of applied voltage.

CONSTITUTION: A power steering device has a pressure control valve, and in this pressure control valve, a piston of a power cylinder is pressed to a right or left side to assist steering force by an assist pressure of oil in accordance with a differential rotational angle between a valve body 6 and a valve shaft 3 which are correlated by a torsion bar 7. Here, electroviscous fluid, which is used as operating oil, is supplied to the pressure control valve through a high pressure oil path 29 from a pump 13. The pressure control valve is constituted so that voltage from a power supply unit 26 can be applied to only the electroviscous fluid positioned between the valve shaft 3 and the valve body 6, and this applied voltage is controlled based on outputs from a car speed sensor 29, flow amount detecting means 20 and rotational angle sensors 22, 23.