PURPOSE: To obtain the production of a transfusion liquid vessel by which a transfusion liquid vessel having a flat containing part can be produced at a high speed by blow molding.

CONSTITUTION: The production of a transfusion liquid vessel 1 has a process of extruding two sheets of parisons opposed to each other; a process of heating the surface of a molded piece 2 which is previously and separately molded as an open part; a process of inserting the molded piece 2 as the open part between the two sheets of parisons; and a process of holding the sheet-form parisons and the molded piece 2 as the open part by a mold and blow molding the vessel 1 in the mold. In addition, the molded piece 2 as the open part is inserted approximately at right angles to the direction of extruding the two sheet-form parisons, and the vessel 1 is continuously blow molded without cutting the sheet-form parisons.