To provide a molding method for a synthetic resin-made tank having uniform wall thickness and satisfactory appearance and shape, capable of being inexpensively molded, and having an allotted barrier film therein.

A fixed mold 1 and a slide mold 10 opened and closed for the fixed mold 1 and driven slidably are used. This synthetic resin-made tank is molded by primary molding in which the allotted barrier films 30, 30 allotted in advance are inserted into cores 5, 15 and melted resin is injected into cavities Ca, Cb constituted by the inserted allotted barrier films 30, 30 and recessed parts 7, 11 of the molds 1, 11 to mold a pair of primary semi-hollow bodies A, B in such a way that its opening parts have join parts and secondary molding in which the slide mold 10 is slid to join the join parts of a pair of primary semi-hollow bodies A, B molded by primary molding and melted resin is injected into a join space part S constituted by joining them to join a pair of primary semi-hollow bodies A, B.