A plug member integrally includes a base portion (35a) attached to a plug holder (31) to be mounted on a mouth portion of a container, a plug (34) engaged with an engagement portion (31c) provided in an outlet passage of the plug holder (31), and a resilient connection portion (35b) resiliently connecting the plug (34) to the base portion (35a). Before the first use, the outlet passage is closed with the plug member. In the first use, the container is squeezed to increase the internal pressure of the container. Thus, the plug (34) is disengaged from the engagement portion (31c) by the internal pressure. The outlet passage is thereafter constantly kept open.
