PURPOSE: To prevent rapid change in the air-fuel ratio, by coupling a cylinder means driven in response to the suction vacuum to a linkage connecting the accelerator pedal and throttle valve co-operatively with each other, and lowering the speed of the movement of throttle valve caused by rapid depression of accelerator pedal.

CONSTITUTION: A linkage for controlling the opening of throttle valve 10b in carburetor in response to the depression of accelerator pedal 1 is constituted by a first and a second bell cranks 4, 6, crank shaft 8 connected to throttle shaft 10 by means of joint 9, connecting rods 3, 5, 7 for connecting them in co-operative manner, etc. A cylinder means S including pistons 13, 14 is coupled to the linkage. Pressure chamber P of this cylinder means S is communicated via hose 15 to vacuum take-out port 16 formed at the downstream side of throttle valve 10b, and piston rod 12 is connected to the first bell crank 4. With such an arrangement, the required effect can be obtained by urging the bell crank 4 in a direction closing throttle valve 10b.