To smoothen the transfer to the speed reduction by detecting the inclination as a degree of curved reduction of a lug cutting width so as to forecast the achievement to the reduction trigger level.

In a displacement curve, inclination (change ratio) is always measured. In this condition, speed reduction control for side trimmer line is started at a time point that a lug cutting width achieves the speed reduction trigger level after (t) seconds (speed reduction control starting point), which is forecasted on the basis of a change of measured change ratio. This speed reduction is performed on the basis of the arrival time from the speed reduction control starting point to an achievement point of the speed reduction trigger level, which is forecasted on the basis of the change ratio of the lug cutting width at the starting point.