A sliding mode control method with feedforward compensation function which realizes control response adapted to varying system parameters and which always normalizes the operation quantity added to the control object. Position deviation (ε), speed deviation (ε), estimated maximum and minimum inertias (Jmax, J0), estimated maximum and minimum gravity loads (GRmax, GRmin), switch-over variable (s), integration element (∫(ε+C.ε)), the second differential (Υr) of an instructed position, and an actual speed (Υ) are periodically calculated based upon the instructed position (Υr), an actual position (Υ), inertia data and gravity load data (100 to 102, 104, 107, 110, 114, 117, 120, 123, 127). Based upon a switch-over quantity (T1) obtained by adding up five terms calculated depending on positive or negative sign of the switch-over variable, position deviation, integration element, second differential and actual speed, a proper torque instruction (T) is periodically calculated as an operation quantity even when nonlinear system parameters such as inertia are changed (105, 106, 108, 109, 111 to 113, 115, 116, 118, 119, 121, 122, 124 to 126, 128 to 131).