PURPOSE: To protect a driving machine from excessive rotation by carrying out forcible hydraulic braking in a case when critical engine speed of the driving machine is excessive and continuing braking until existing downhill driving force becomes smaller than receivable output by the driving machine.

CONSTITUTION: Forcible hydraulic braking is carried out in the case when critical engine speed of a driving machine is exceeded on a vehicle furnished with at least one adjustable hydrostatic transmission mechanism and a braking device at least partially and hydraulically reducing kinetic energy of a vehicle at the time when the maximum receivable torque is exceeded by the relative driving mechanism in the case when the driving machine receives torque. Thereafter, this braking makes existing downhill driving output smaller than output receivable by the driving machine or continue until it becomes speed equal to the output.