There is provided 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. The plug member has a spherical seal surface with which the elastic membrane element elastically and closely contacts. At least one thick wall portion and at least one thin wall portion are alternately formed around the discharge opening of the elastic membrane element in a circumferential direction thereof.