A method of sliding mode control for realizing a control system which exhibits strong robustness and excellent control stability. A processor of a digital servo circuit of a servo control system includes a position loop that effects proportional control, a speed loop that effects proportional-plus-integral control to calculate a torque instruction, and a current loop that responds to the torque instruction. The processor calculates a position deviation (x1), the differential (x2) of the position deviation, the integral (x3) of the differential of position deviation, and the integral (x4) of the position deviation (100 to 103) based upon an instructed position (Υr), the actual position (Υ) and actual speed (Υ), and further calculates a switching surface (s = x2 + C.x1 + K.x3 + K.Kp.x4) in the sliding mode control and a discrimination reference value (Kp.x1 + x2). The processor further calculates switching quantities ((τ', τ') that enable the characteristics of the control system to be converged on the switching surface depending on the switching surface, position deviation and discrimination reference value of positive or negative sign, and the motor is operated according to a torque instruction (τ = τ' + τ') obtained by adding them up together. The robust control system controlled by the sliding mode does not permit the characteristics to change even when the motor inertia undergoes a change.