Injection molding die and injection molding method

A die is formed of a cavity mold having a hollow portion and a slidable core mold that allows a core to be inserted into hollow portion of cavity mold. An insert part that has been previously molded is fitted to and supported by the front end of core of core mold. Then, insert part is inserted into die and clamped therein, then a molten molding material is injected into die so as to perform injection molding of a container body of a substrate storage container with its rear part formed with insert part. Since insert part is put into contact with hollow portion of cavity core so as to restrain vibration of tapering flat core, core of core mold will not be flapped or flexed even if difference in injection pressure occurs in the molding material.