PURPOSE: To increase pump drive resistance and effectively prevent an acceleration slip by providing an open/close valve at the delivery oil passage of an engine driven oil pump for a power steering device, and closing the valve upon detection of the acceleration slip.

CONSTITUTION: A hydraulic power steering device 20 supplies pressure oil sucked from an oil sump 22 via the operation of an engine driven oil pump 21, to a control valve 24 operated with a handle 25. Furthermore, the device 20 supplies the pressure oil to a power cylinder 27 via a control valve 24 for assisting steering power. In this case, an electromagnetic valve 23 is provided in an oil passage between the pump 21 and the control valve 24. Also, when the acceleration slip of a wheel is detected in an electronic control device 12 on the basis of a wheel speed detected with each of speed sensors 13 to 16, the electromagnetic valve 23 is closed only upon detection of the acceleration slip, and the drive resistance of the pump 21 or the power load of an engine 1 is increased, thereby quickly decreasing the number of engine revolutions.