PURPOSE: To freely determine a duty ratio over a low speed region and a high speed region of engine so as to facilitate air-fuel ratio control without degrading durability or increasing a cost.

CONSTITUTION: In an engine air intake control device with which air is supplyed to intake passages of carburetors 10La, 10Lb via duty solenoid valves 12 and an air supply quantity is controlled by adjusting duty ratios of the solenoid valves 12 so as to control an air-fuel ratio, a control unit 22 is provided for changing operation frequencies of the duty solenoid valves 12 based on an engine speed.