To hold airtightness between the taper face of an annular rib provided by the extending downward of the inner peripheral wall of an annular groove in a main body made from a resin and a chamfered part at the opening of a valve stem by making them abut to each other.

In a valve cap made from a resin, in which an internal thread 2 that meshes with an external thread on a valve stem 5 is provided inside a cylindrical main body 1 made from a resin and an annular groove 4 into which the end of the valve stem 5 fits is provided on the outer edge of a bottom surface 3, the inner peripheral wall surface of the annular groove 4 formed at the outer edge of the bottom surface 3 is extended downward from the bottom surface to suspend an annular rib 4a, and a taper surface 4b is provided on the outer periphery of the lower end of the annular rib 4a, so that by making the taper surface 4b of the rib abut to the chamfered part 5a of the valve opening part of the valve stem 5 during installation of the valve cap, they can be held airtight.