PURPOSE: To offset a drift or a nonlinearity by estimating the current offset of a servo motor by obtaining the offsets of current detecting systems of phases based on an integrated value and estimating the current offset of the motor.

CONSTITUTION: Integrated values (a), (b) by one period of an electric angle θand a counted value CNT for counting the number of times of a speed loop are set as initial values, a FLAG 2 is set to '0' and a FLAG1 is set to '1' (S1-S3). Then, the content of the FLAG2 is decided, and when it is set to '0', the angle θ at that time and a corresponding torque command T(θ) are read (S4). Thereafter, a calculation is conducted by using the read angle θ and the command T(θ). Then, '1' is added to the value CNT, and when the obtained value CNT satisfies 'N' and the integration of one period is finished at the angle, offset amounts Ior, Ios and Iot are obtained, and an offset compensation is conducted (S5-S9).