PURPOSE: To prevent a steering wheel from being excessively turned during running on a wet road under raining, by determine a position of a power switch so as to determine an instruction current value delivered to assist motor in accordance with a steering torque and a vehicle speed.

CONSTITUTION: A controller 10 which is connected thereto with a wiper switch 1, controls a current value for an assist motor 56 so as to adjust the motor current value in accordance with a vehicle speed, a steering wheel manipulating torque, a position of the wiper switch 1 and an ambient temperature. Further, in a fine weather condition in which the wiper switch 1 is turned off, the motor current value becomes highest. The motor current value is successively decreased in the order of a sprinkling condition (the wiper switch 1 is set to a position INT), a slight rain condition (the wiper switch 1 is set to a position Lo) and a heavy rain condition (the wiper switch is set to a position Hi). Thus, the steering wheel is turned up to a position where the resistance against the manipulation of the steering wheel becomes usual, and accordingly, wheels are set to a substantially constant steered angle, thereby it is possible to prevent the steering wheel from being excessively turned.