Mouth plug, packaging container, and method for manufacturing mouth plug

A mouthpiece according to one aspect of the present invention has a spout, a cap, a strap portion, and a rib portion. The nozzle has: a cylindrical side wall; an external thread; and a flange provided on one end side of the side wall. The cover has: a top plate; the peripheral wall is connected with the outer peripheral edge of the top plate; and an internal thread screwed with the external thread. The band portion is connected to one end of the peripheral wall and fixes the cap to the spout between the peripheral wall and the spout. The rib portion connects the peripheral wall and the band portion, and protrudes toward the inside of the cover from the inner peripheral surface of the end portion of the peripheral wall and the inner peripheral surface of the band portion. The spout also has a protrusion provided on the side wall and restricting movement of the cap and the strap portion with respect to the spout. The band portion and the peripheral wall are defined by a kerf located above a portion of the protr