PURPOSE: To prevent the life of a chassis from being reduced and as well as prevent it from being damaged by a method wherein the stress applied to the chassis is detected and, when said detected stress exceeds the predetermined stress level, the vehicular speed is limited.

CONSTITUTION: A maximum speed setter 2 sets he maximum speed corresponding to the detection signal, which is outputted in accordance with the stress of the chassis from a stress detector 1, and outputs it to a controlling circuit 3. When the actual vehicular speed detected with a speed detector 5 exceeds the set maximum speed, the controlling circuit 3 outputs the signal to an electric-to-hydraulic pressure converter 7 and said converter 7 actuates a hydraulic pressure-to-hydraulic pressure converter 14 so as to apply a retarder brake 15. At this time, the braking is applied so as not to give the sense of discomfort to a driver by means of an acceleration setter within the controlling circuit 3. Furthermore, when the oil temperature value of the brake detected with an oil temperature detector 20 exceeds the set maximum temperature, the brake 12 is released by means of the controlling circuit 3. In addition, the controlling circuit 3 controls an overturning angle controlling mechanism 29 in accordance with the speed change signal of a controller 27 so as to send the amount of cooling oil suitable for the changed speed stage from a pump 28 to the brake 15.