PURPOSE: To reduce a cost required for adjustment and to simplify adjusting work by controlling the increase and decrease of intake air quantity through an adjusting signal generated according to deviation of the number of the revolution of an engine and aimed number of the revolution and also, independently of this, and providing an adjusting means capable of increasing and decreasing the intake air volume.

CONSTITUTION: An error Δn is calculated by an error amplifier 61 through an engine speed Ne by an engine speed sensor 42 and output Nt from a target engine speed generator 5, according to the engine speed error Δn, an engine speed correction signal Sc is generated in an engine speed adjustor 62. Next, the output ST + Sc of the engine speed adjustor 62 is applied to a limiter 12 and output Y proportional to input X is generated in the limit of Xmin < X < Xmax, and beyond the limit, the output Y is limited by Xmin or Xmax. The output of this limiter 12 is changed to the driving signal of a solenoid valve 8 as an intake control valve by a driving device 7 and intake air quantity is duty-controlled. At this time, the intake air quantity is adjusted by an air adjusting screw 4 to show an meter indication corresponding to duty 50%.