a main housing having an inlet port defined therein for introducing said reaction gas, an outlet port defined therein for discharging the reaction gas introduced from said inlet port, and a communication chamber defined therein and interconnecting said inlet port and said outlet port;

a solenoid disposed in a solenoid housing connected to said main housing, for being energized by an electric current supplied thereto;

a movable member disposed in confronting relation to a fixing member disposed in said solenoid, for axial displacement upon energization of said solenoid;

a valve head having a first valve being sealable on and away from a first valve seat mounted on said movable member and a second valve being seatable on and away from a second valve seat on said main housing;

a flexible member disposed in said communication chamber and connected to said valve head, said flexible member being flexible upon displacement of said valve head, said flexible member dividing said communication chamber into a first communication chamber and a second communication chamber; and

a communication passage defined in said main housing and providing fluid communication between said first communication chamber and said second communication chamber, said communication passage including a restriction for reducing the rate of the reaction gas flowing through said communication passage;

a removing member disposed in said inlet port for removing dust particles contained in said reaction gas, said removing member including a plurality of fine pores having a pore size smaller than a diameter of a passage defined in said restriction;

a first spring disposed between said second valve and an engaging member disposed in said main housing for biasing said second valve to move in a direction away from said second valve seat; and

a second spring disposed between said movable member and said solenoid for biasing said movable member to move in a direction away from said fixing member by resilient force,

wherein said valve head has a guide extending from said second valve in a direction away from said first valve, said guide is axially displaceably guided in said main housing, and

wherein said main housing has a hollow cylindrical guide sleeve, said guide being axially displaceably held by said hollow cylindrical guide sleeve, said hollow cylindrical guide sleeve being axially engaged by an engaging member mounted in said main housing.