To provide such a valve element construction that a valve is maintained closed up to a move distance (lift) of the valve element, corresponding to a low opening degree in a conventional method by which chattering easily occurs, and the valve is opened when exceeding a distance separated from a valve seat in which chattering does not occur in the valve element.

The valve element 10 is provided to be able to seat on and unseat from the valve seat 9 provided in one opening end of a valve port 8 communicating with a primary side flow passage 5 and a secondary side flow passage 6. The valve element 10 includes such a sliding valve 10b that the valve is maintained closed between a seating position from the valve seat 9 and a position Y separated therefrom by a predetermined distance L, while coming into slidable contact with a valve port inner wall 8a and being reciprocatably inserted into the valve port 8, and the valve is opened by being separated from the valve port 8 when exceeding the position Y.