A spout assembly with a spout and a cap. The spout has a cylindrical sidewall; a cylindrical base at the lower end of the sidewall and having a larger outer diameter than the sidewall; and a disk-like flange extending outward from the lower end of the base. The cap has a cylindrical peripheral wall with an internal thread and is screwed onto the upper end side of the spout sidewall. The spout sidewall also has a projection formed between the external thread and the recess so as to extend circumferentially and have a height lower than a height of the external thread, and with the cap screwed onto the spout, at least a portion of an upper surface of the base fits an inner side of a peripheral wall of the cap.