PURPOSE: To enable fine adjustment of air suction amount in low load area by relatively enlarging change amount of throttle valve opening to change amount of accelerator pedal step-in amount in the larger area of step-in amount.

CONSTITUTION: A throttle valve opening sensor is installed on a position where the signal proportional to the step-in amount of an accelerator pedal, and a mechanical coupling device coupling the accelerator pedal and the throttle valve is constituted so that the degree of throttle valve opening change to the change amount of step-in amount of the accelerator in the larger accelerator pedal step- in amount area becomes larger than that in the smaller accelerator pedal step-in amount area. Thus, the fine adjustment of suction air amount can be made in the low load operation area.