Turning radius is calculated from steering angles of steering-wheels (2, 3) and articulation angles of respective frames, and the steering-wheels (2, 3) are automatically rotated faster than rear-wheels (4, 5) based on the turning radius, the revolution number of the rear-wheels (4, 5), engine speed of an engine (6) and a speed range of hydraulic motors (14, 15), so that conventional trouble for manipulating operation lever etc. while turning a motor grader (1) and skill-requiring work for adjusting the revolution number of the steering-wheels (2, 3) in accordance with the revolution number of the rear-wheels (4, 5) can be eliminated, thereby easily and accurately controlling the revolution number of the steering-wheels (2, 3) and securely preventing tight-corner braking phenomenon.