To open a valve appropriately at an intended pressure or at an approximately intended pressure even in the case of being used over a long period of time by hardly making adhesion of a valve element and a valve seat, in the valve.

The valve is closed by pressing a protruding part 42 provided at the valve seat 34 to a valve element main body 38 of an elastic material such as rubber. In a casing 30 having a valve seat 34, a stopper 44 is fixed to a corner part around the protruding part 42. A face in an axial direction of a stopper 44 is provided to face against another face in the axial direction of a sliding body 40 fixed to the valve element main body 38 via a clearance. The stopper 44 regulates displacement of the valve element main body 38 to the valve seat 34 due to biting of the valve seat 34 and the valve element main body 38.