PURPOSE: To reduce NOx while preventing smoke from worsening by preventing control accuracy of exhaust gas recirculation control from decreasing due to collapsing a correlation between duty ratio and negative pressure of a negative pressure control solenoid valve.

CONSTITUTION: A duty control type negative pressure control solenoid valve 18 is provided so as to variably control an opening of an exhaust gas recirculation (EGR) control valve. In order to detect a new air flow amount varied in accordance with an EGR amount, an air flowmeter 23 is provided, and based on a value obtained from a map by a detection value of the flowmeter and an operative condition, required duty ratio is set. The required duty ratio, when it is in a prescribed range, is left as output, but in the case of exceeding a lower limit value or an upper limit value, the duty ratio is fixed to the lower and upper limit values. For compensating overs and shorts of the EGR amount according to the above, injection timing of a fuel injection pump 20 is corrected.