A flexible package bag is equipped with a film-shape non-return nozzle by which a pouring amount of a liquid packed material can be stabilized by rectifying a pouring flow of the liquid packed material. The pouring performance and breaking resistance of the bag can be improved by reinforcing the film-shaped non-return nozzle. A flexible package bag is formed by fusion-joining a base end portion of a film-shaped non-return nozzle having an outer non-return function of preventing penetration of ambient air into the bag to inner surfaces of a side portion of a main package bag body composed of plastic laminate films and projecting therefrom. A seal section for rectification/reinforcement formed by fusing two opposed plastic laminate films is disposed at a position near to the main package bag body including a lower portion of a fused section to the base end portion of the film shaped non-return nozzle.