Patent ID: 7490744

Claim:
A backflow preventing mouth plug comprising: an elastic membrane element having a discharge opening for discharging fluid and mounted to a mouth part of a container and a plug member disposed in the elastic membrane element to prevent unintentional outflow of the fluid, wherein the plug member has a spherical seal surface with which the elastic membrane element elastically and closely contacts, wherein the elastic membrane element has an inner surface in close contact with the spherical seal surface of the plug member, and an opposite outer surface facing away from the spherical seal surface of the plug member, wherein the discharge opening is located at a projection end of a tip part of the elastic membrane element, wherein a plurality of thick wall portions and a plurality of thin wall portions are alternately formed after each other and around the discharge opening of the elastic membrane element in a circumferential direction thereof, and wherein each of the plurality of thick wall portions has a shape of a protuberance that is bulged outward from the outer surface of the elastic membrane element, and is formed closer to the discharge opening than a spherical center of the plug member.