PURPOSE: To reduce energy loss and prevent the allowable number of revolution from being exceeded by obtaining a simple circuit which changes the car-speed instruction for controlling the speed of a hydraulically-driven car in accordance with the deviation between the set revolution number and an actual revolution number.

CONSTITUTION: When a car descends a slope in the state where a car-speed setting lever 2 and a throttle valve 4 are perfectly opened, the number of revolution of an engine increases. When the high idle set by the lever 4 is set at 2,300rpm, and the allowable revolution number of the engine is set at 2,500rpm, if 2,300rpm is exceeded, the signal supplied from a revolution sensor 5 and an FV converter 42 becomes larger than the signal supplied from a potentiometer 23, and the speed change signal S1 which is gradually reduced from '1' through an absolute- value circuit 43 and a function generating circuit 44 is added into a multiplication device 50, and a car-speed instruction signal Vs is gradually reduced to zero at 2,500rpm. Thus, the need of installing a special hydraulic circuit for preventing over-revolution of the engine can be obviated, and also energy loss can be reduced.