a second valve seat 26 made of synthetic resin in annular shape and being embedded in the second valve chamber surface at the bottom of the second valve chamber 25 a second diaphragm 27 made from thin metallic sheet such as stainless steel or inconel trade name positioned in the second valve chamber 25 for maintaining airtightness in the second valve chamber 25 and for contacting and moving away from the second valve seat 26 as a center portion of the second diaphragm 27 respectively moves up and down a second bonnet 28 made from metal in cylindrical shape and inserted in the second valve chamber 25 to hold an outer circumference of the second diaphragm 27 airtight against the body 2 a second bonnet holder 29 to push the second bonnet 28 toward the body 2 bolts not shown to secure the second bonnet holder 29 to the body 2 a second stem 30 made of metal and supported for vertical movement on the second bonnet 28 a second diaphragm holder 31 at a top tip of the second stem 30 to contact a center portion of the second diaphragm 27 and a second spring 32 to cause the second diaphragm 27 to contact the second valve seat 26 by pushing the second stem 30 upwardly a second cover 33 to generally cover the second.