Abstract: A backflow preventing plug, a container, and a pouring device in which a container body and a plug member are provided separately in a simple shape, and tight-contact property between the plug member and a resilient member is enhanced to improve a backflow preventing function are provided. A backflow preventing plug 4 includes a resilient membrane 7 formed of a thin film for defining a flow path having an inlet port 9c and an outlet port 6, and a ball-shaped plug member 8 retained in the flow path of the resilient membrane 7 and formed with a spherical sealing surface 12 for restricting fluid flow by coming into tight resilient contact with a middle portion of the resilient membrane 7. In a container 1, the backflow preventing plug 4 is attached to a container opening 2 of a container body 3 accommodating contents.