A structure of a mouth plug portion includes a nozzle and a cap, the nozzle includes a first outer protrusion and a second outer protrusion, the first outer protrusion includes a locking surface that locks the cap and a first guide surface spirally extending, the cap includes a cap body and a tamper band provided with a weak portion that can be broken by an external force received from the second outer protrusion during rotation of the cap body, the cap body includes a first inner protrusion and a second inner protrusion, an outer peripheral surface of the nozzle is provided with a second guide surface along which the first inner protrusion and the second inner protrusion can move in an axial direction.