PURPOSE: To hold a brake power to the magnitude corresponding to the brake pedal stepping-on force by prohibiting the introduction of the intake negative pressure to a negative pressure actuator in brake application when a control valve for controlling the operation of an engine is driven by the negative pressure actuator operated by the intake negative pressure.

CONSTITUTION: A control valve 18 for controlling the operation of an engine varies the characteristic frequency of an intake system by opening and closing the control valve 18 which is installed at the connection part between an independent intake passage 11 for each cylinder and a communication passage 17 which allows the independent intake passages 11,... to communicate each other on the upstream side of an injector 16. The control valve 18 is operated by introducing the intake negative pressure into a negative pressure actuator 20, in the prescribed operation state of the engine. In this case, a brake means B having a brake power increasing means 35 for increasing the brake power for a vehicle by introducing the intake negative pressure is installed. When the brake application for the vehicle by the brake means B is detected by a brake detecting means 65, the introduction of the intake negative pressure for the negative pressure actuator 20 is prohibited by a prohibiting means 66.