PURPOSE: To control an idle speed without causing the stop of an engine, by providing a pressure responsive type actuator controlling the idle speed and a solenoid valve regulating control pressure input to the actuator and controlling said valve in accordance with an idle operational condition.

CONSTITUTION: The captioned controller is operated such that a solenoid valve 21 is opened to supply negative pressure of a negative pressure generator 6 to an actuator 4 through a negative pressure passage 8 and move a stopper 5 leftward. Then a swivelling arm 3 is swung to turn a control lever 2 of a fuel injection pump 1 in the fuel increasing direction and increase an idle engine speed. Here the valve 21 controls its opening in accordance with a deviation between a target idle speed and an engine speed by a speed sensor 24 in a control circuit 22 when idle speed time is decided from an output of a throttle sensor 26 and an auxiliary switch 25 is turned ON or an output of a water temperature sensor 23 is a prescribed value or less.