To provide a synthetic resin cap capable of reducing plug opening torque, while keeping high sealing performance.

A synthetic resin cap is provided with a top wall 5, and a skirt wall 6 suspended from the outer circumferential part of the top wall 5, and is threadedly fitted to a container opening portion, annular inner legs 7 contacting closely with an inner circumferential face of the container opening portion are adjacently provided in an inner face of the top wall 5, an outer circumferential face of an upper part 11 of the inner leg 7 is expanded diameter toward a lower side and an outer circumferential face of a lower part 12 of the inner leg 7 is contracted diameter toward the lower side, to make an outside diameter of the inner leg 7 maximum in the central part 10 of the inner leg 7, and an outer circumferential face of the central part 10 is suspended downwards, is expanded diameter toward the lower side at a degree more moderate than the outer circumferential face of the upper part 11, or is contracted diameter toward the lower side at a degree more moderate than the outer circumferential face of the lower part 12.