A valve mechanism includes a first plunger, a second plunger, a seal member, and a valve element. The first plunger has a thread engaging with a threaded portion provided on an inner wall of the flow passage. The first plunger is movable along the axis by rotating around the axis. The second plunger has a first end and a second end. The first end engages with the first plunger such that the first plunger is relatively rotatable with respect to the second plunger. The second plunger is movable along the axis by rotating the first plunger. The seal member is provided to surround the second plunger. The valve element has a back end engaging with the second end such that the valve element is movable along the axis according to movement of the second plunger so as to be seated on and separated from a valve seat.