PURPOSE: To prevent hesitation of acceleration irrespective of whether warming-up of an engine is completed or not, by disposing a first vacuum outlet port and a second vacuum outlet port for introducing negative pressure into an actuator for intake throttle valves respectively just on the downstream side of the position where a first throttle valve is located when it is closed completely and just on the downstream side of the position where the first throttle valve is located when it is slightly opened, and using the two vacuum outlet parts selectively.

CONSTITUTION: A first throttle valve 2 is disposed in a first intake passage 1a of a fuel- injection engine while a second throttle valve 6 and a control valve 10 are disposed in a second intake passage 1b. A diaphragm means 11 is connected to a lever 12 of the control valve 10, and a first and a second vacuum outlet ports 18, 19 are communicated with a vacuum chamber 15 of the diaphragm means 11 via a changeover valve 20. The first vacuum outlet port 18 is disposed just on the downstream side of the position where the first throttle valve 2 is located in the state that it is opened to a prescribed angle before warming-up of the engine is completed, while the second vacuum outlet port 19 is disposed just on the downstream side of the position where the first throttle valve 2 is located in the state that it is closed completely after warming-up of the engine is completed. With such an arrangement, negative pressure is always introduced into the vacuum chamber 15 irrespectively of whether warming-up of the engine is completed or not. Therefore, it is enabled to prevent hesitation of acceleration by operating the diaphragm means at the time of accelerating the engine.