PURPOSE: To prevent leakage between a valve body and a valve seat by increasing the adhesion of the valve body and the valve seat by utilizing the differential pressure between an inflow passage and an outflow passage, in the condition where the valve is closed.

CONSTITUTION: A valve chamber 12 is formed of a valve box 11 and a lid 14, and an inflow passage 16 is provided to communicate from the outside of the valve chamber 12 to the inside of the valve chamber 12. Also an outflow passage 17 is provided to communicate from the inside to the outside of the valve chamber 12, and a valve seat 18 is formed around the intake of the outflow passage 17. A valve body 19 is provided in the condition to move up and down freely opposing to the valve seat 18 inside the valve chamber 12, and a valve rod 23 is provided to the upper side of the valve body 19 in the condition to move up and down freely to a penetrating hole 21 provided to the lid 14 through a sealing member 22. A driver 51 to drive the valve rod 23 is provided at the end of the valve rod 23 at the opposite side of the valve body 19 at the upper side of the lid 14.