PURPOSE: To prevent engine stop by operating an auxiliary speed-controller in a nonworking state and reducing the engine load in case of sudden loading on the engine.

CONSTITUTION: In detection of a nonworking state by a working-state detector (S0), an auxiliary speed-controller 101 is operated and in case of [a rotational frequency detected by a rotational frequency detector (S1)]≤(the standard rotational frequency for the nonworking state), a speed change gear 9 is automatically operated to reduce the car speed for reduction of the load of the engine (E).