A plurality of position data sets representing a plurality of points on a road ahead of a vehicle are acquired, and the degree of curvature of the road at each point is computed. On the basis of the degree of curvature, a constant curvature degree section of a curve is identified, and the degree of curvature and the end position of the constant curvature degree section are determined. In order to cause the vehicle to properly pass through the curve, curve deceleration control is executed on the basis of the actual vehicle speed, a proper vehicle speed determined from the degree of curvature, and the end position of the constant curvature degree section. That is, the curve deceleration control is performed on the basis of the start point of a section of a curve having the maximum degree of curvature and the constant degree of curvature of that section.