Characteristic lines A to D of lateral acceleration G-required torque T in determination of required torque T are used as switched according to increase/decrease of the absolute value of the lateral acceleration, and the direction thereof. The lines A, C used in increase of the absolute value of the lateral acceleration G are so set that the absolute value of required torque T becomes larger at the same lateral acceleration G (G.noteq.0) than that with the lines B, D used in decrease of the absolute value of the lateral acceleration G. The difference between line A and line B (the difference between line C and line D as well) at the same lateral acceleration G is so set that the difference near the lateral acceleration of 0 is smaller than that in the other regions. Particularly, it is preferable to set the required torque T to 0 at the lateral acceleration G of 0.