PURPOSE: To enhance production efficiency by fusing a plugged lid or flanged adaptor ring formed from a plastic material by injection molding to the free edge of a cask body.

CONSTITUTION: A closed cylindrical duplex body 8 is subjected to blow molding so as to become a flexible tubular preform subjected to extrusion molding within a blow mold 6 to be taken out of the mold 6 and, thereafter, an intermediate ring 9 containing the blow opening 10 of a thrust blowing needle 7 is cut off to divide the body 8 into two cast bodies 8a, 8b comprising a thermoplastic material and having the same size. When a plugged cask is produced, plugged lids formed from a thermoplastic material by injection molding are fused to the upper edges of the cask bodies 8a, 8b. When a cask having a detachable lid and a tensioned ring-shaped plug and having a largely opened neck is made, flanged adaptor rings formed as plastic injection molded members are fused to the upper edges of the cask bodies 8a, 8b in order to arrange lid flanges.