To retain a prescribed opening/closing action by suppressing to the minimum deterioration of opening/closing characteristics to be caused by swelling of an elastic polymer valve piece provided in a valve element of an opening/ closing valve due to handling fluid.

A base part 13 of the elastic polymer valve piece 12 is fitted in the bottom of a fitting hole 6 of the valve substrate 1 of the valve element with a space formed by a small diameter hole part left therein and a contraction part 10 formed in the tip opening part of the fitting hole 6 is bitten and fixed therein. The axial extension by the swelling of the valve piece 12 is dispersed into a frontward extension and a rearward extension of the fixing part fixed by the contraction part 10 so as to reduce the axial extension of the tip part 14 with an abutment part 15 and reduce the deviation from the set value of the timing of the opening/closing.