PURPOSE: To prevent the aggravation of exhaust particulates and smoke caused by excessive exhaust gas recirculation in a non-idle low load region by performing a large quantity of exhaust gas recirculation only at the actual idling time of an engine.

CONSTITUTION: A lever opening sensor formed of a potentiometer is provided to detect the opening of the control lever of an injection pump, and the maximum exhaust gas recirculation is performed when the output voltage is the first opening judgment level T1 or less. The first opening judgment level T1 is set as T1=x+K.σ (S32) by numerously sampling the actual sensor voltage ICLV at the idling time (S23-S28), obtaining the average (x) and dispersion σ(S29, 30) and using a coefficient K corresponding to the sample number (S31).