PURPOSE: To improve the reliability in steering by constituting an aimed steering angle signal generating means so that the output signal of a rotary encoder for detecting the revolution direction and revolution quantity of a steering wheel is counted within a certain upper limit value and lower limit value and converting the counting value into the analogue signal and outputting the counting value.

CONSTITUTION: An aimed steering angle signal generating circuit is equipped with a rotary encoder 2 consisting of a rotary disc on which a number of slits are arranged in the circumferential direction and an interruption type optical sensor for detecting the slits. The revolution direction of a steering wheel 1 is discriminated from the output signals of two optical sensors having the phase different by 90°, and the signal U/D representing the revolution direction of the steering wheel 1 is inputted into a counter 4, and the counting of the output signal C by the counter 4 is carried out. Then, after the counting value of the counter 4 is converted into the analogue signal by a D/A converter 5, the count value is inputted into a servocontroller through a selector switch 6, and the steering wheel is turned by controlling the drive of a hydraulic cylinder.