To eliminate a steering angle detection means from an automatic steering device with an electric motor serving as an actuator, and calculate a steering angle of a wheel based on a rotational angle of the electric motor.

In a model of an electric motor, when assuming (u) for terminal voltage, (i) for carried current, R for resistance, L for inductance, α for rotational angle, and K for constant determined by a characteristic of the electric motor, the terminal voltage (u) is given by a formula u=L(di/dt)+Ri+K(dα/dt). This rotational angle is solved by α, when induction L of small value is omitted, the rotational angle α is given by a formula α=(1/K)∫(u-Ri)t. Since the rotational angle α of the electric motor and a steering angle of a wheel are in a proportional relating, when the rotational angle α is calculated from the terminal voltage (u) of the electric motor and the carried current (i), the steering angle of the wheel can be known. In this way, a steering angle detection means can be eliminated.