such as styrene butadiene rubber made integral therewith by vulcanization adhesion the heat resistant rigid vinyl chloride resin is chlorinated polyvinyl chloride resin composition of 34 type iv grade 1 34 provided in astm d 1784 which has heat distortion resistance temperature not less than 100 u00b0 c the upper end of the valve body 2 is provided with projections 2a and 2a 39 which slidably engage with guide grooves 1a and 1a 39 of the valve casing 1 so as to prevent the valve body from rotation thereof and to allow the valve body only to move up and down the bottom of the valve body 2 i e the lower end of the valve body 2 is spherically shaped the diameter of the valve body 2 increases toward the top end thereof the taper ratio is preferably about from 1 10 to 1 5 the base end of the valve body 2 i e the upper end of the valve body 2 is provided with an internal thread 2b at the center thereof a hollow portion 2c leading to the internal thread 2b is formed thereunder the internal thread 2b is screwed on an external thread 3a of a spindle 3 extending in the valve chamber 10 as shown more clearly in fig2 the center portion of the bottom of the valve body 2 is provided with a small hole 2d communicating with the hollow portion 2c of the valve body 2 and having an internal diameter less than that of the hollow portion 2c the upper portion of the outer surface of the valve body 2 is provided with an annular fitting groove 2e the small hole 2d and groove 2e are not always necessary but they are preferably.