A mouthpiece structure for a pressure vessel includes a liner made of a synthetic resin, and constituting a vessel body and a cylindrical part communicating between an interior and an exterior of the vessel body, a cylindrical mouthpiece mounted closely to an inner periphery of the cylindrical part, a retainer having a covering portion which covers regions of outer surfaces of the cylindrical part and the mouthpiece respectively and a peripheral wall surrounding the cylindrical part, the cylindrical part and the mouthpiece having an interface between an inner periphery of the cylindrical part and an outer periphery of the mouthpiece, the interface having a distal end facing the regions, a first sealing member disposed between the mouthpiece and the covering portion, and a second sealing member disposed between the cylindrical part and the peripheral wall of the retainer.