PURPOSE: To aim at simplifying and miniaturizing a system for purifying an engine exhaust gas, by integrally incorporating a vacuum regulator and a duty control solenoid valve.

CONSTITUTION: A valve member 23 is moved to the left by a spring 24 when a solenoid coil 17 is deenergized, and therefore, the communication between a negative pressure input port 13 and a negative pressure chamber 28 is interrupted while the communication between a negative pressure output port 14 and an atmospheric air intake port 12 is established. When the solenoid coil 17 is energized in this stage, an iron core 18 into which threaded member 19 is screwed, generates attractive force which causes a movable core 20 and a holding member 22 to be moved to the right together as one unit, overcoming the spring 21. At this time, the valve member 23 is also moved, accompanied with them, and therefore, the communication between the ports 13, 14 is established. When the value of the negative pressure introduced in the negative pressure chamber 28 reaches a predetermined value which is determined by an input current, a diaphragm 26 is moved to the left, thereby the valve member 23 is again held in its close position.