PURPOSE: To increase the amount of EGR in proportional to the increase of a load and achieve the reducing of NOX with a high efficiency by a method wherein a vacuum which is inversely proportional to the vacuum of a suction pipe is given to the diaphragm chamber of a pressure regulating valve.

CONSTITUTION: An inversely proportional type vacuum control valve 42 is connected to the pressure regulating valve 28 to give the vacuum in inverse proportion to the vacuum of the suction pipe to the diaphragm chamber 32 of the pressure regulating valve 28. The diaphragm chamber 32 of the pressure regulating valve 28 and the second vacuum port 44 formed in the suction path 2 at the downstream side of the throttle valve 8 of a carburettor are communicated through a second vacuum path 46, the inversely proportional type vacuum control valve 42 is interposed in the second vacuum path 46 and the A port 48 thereof is connected to the side of the second vacuum port 44 while the B port 50 thereof is connected to the side of the diaphragm chamber 32 of the pressure regulating valve 28 respectively. According to this method, the output vacuum between -100W-400mmHg in the set range of a signal vacuum may be reduced in inversely proportionally.