Abstract: This invention is related to a supplemental mouthpiece which is adapted to be mounted on the mouth portion of a liquid bottle and provided with a partitioning strip extending into the interior of the bottle. The cap for the bottle can be mounted while the supplemental mouthpiece is left attached to the mouth portion. According to this supplemental mouthpiece, the partitioning strip is mounted on the mouth portion to partition the interior thereof and one of the partitioned passage is covered by a cover provided with a vent hole to prevent the splashing of liquid and at the same time to prevent the liquid from pouring out from the covered passage. The cover may be formed integral with the mouth portion, or with a securing member which is adapted to be mounted on the mouth portion, thereby allowing the cover to be detachably mounted on the mouth portion.