the lower end portion thereof based on this arrangement as shown in fig9 2 the lower end portion of the valve rod 2 is provided with a stepped surface 22 which is located at a position higher than that of the opening groove 3 and further the stepped surface 22 may be provided inside of the lower end portion of the valve rod 2 based on this reference is also made to fig9 1 to fig9 3 the sleeve protrusion portion 122 may be further extended into the lower end portion of the valve rod 2 or be moved out of the valve rod 2 such that the upper end face of the sleeve protrusion portion 122 is hermetically contacted with the stepped surface 22 or is disengaged from the stepped surface 122 this kind of structure arrangement has the same technical effect as that of the structure arrangement in which the opening groove 3 is provided on the sleeve protrusion portion 122 as shown in fig4 and fig5 which will not be described in detail moreover in this kind of structure arrangement the upper end face of the sleeve protrusion portion 122 is connected with the stepped face 22 in a sealed manner and the stepped face 22 is not easy to be deformed because of its strong rigidity therefore compared with the structural arrangement of the sealing member in the prior art the sealing performance and service life of the.