Abstract: A valve seatis mounted within a housing. An actuating shaft movable in the axial direction is rigidly fixed to center portions of an elastic valve and snap-action plate. The elastic valve has an outer resilient peripheral portion press-fitted to the valve seat by it's elasticity and separates the interior of the housing into a low and a high pressure chamber. The snap-action plate responds to the movement of the actuating shaft and rapidly changes a pair of contacts responsive to the movement of the actuating shaft from one position to the other position. The low pressure chamber is connected to an intake manifold of a combustion engine and high pressure chamber is opened to the atmosphere. When the overpressure is produced in the low pressure chamber, the peripheral portion of the elastic valve separates from the valve seat and performs protection.