Disclosed is a bag container (10) wherein a synthetic resin container body (11) equipped with a pouring port forming portion (13) and a filling port (30) projected therefrom is formed by a molding, and the filling port (30) is sealed by fusion after filling a content therefrom. The filling port (30) has a square cross-section at a sealing base portion (32), and a tip side section change portion (34) and a cylindrical portion (33) are provided closer to the tip side than the sealing base portion (32). Triangular panel portions (35) of isosceles triangle having each side of the square cross-section as a base (35a) are provided at four positions closer to the container body (11) side than the sealing base portion (32) continuously in the circumferential direction of the filling port (30). When the filling port (30) is sealed by fusion, the region from the sealing base portion (32) to the tip side is folded flatly while arranging a pair of corners (36) facing each other at the opposite ends.