PURPOSE: To obtain a fabrication technique for a waterproof ballast case which is less expensive and can contribute to the improved workability thereof by forming, on the bottom plate of a cylindrical case, a dam which is in proximity to a folded joint, which is taller than the joint, and which is parallel with the side wall of the case, thereby forming a channel between the dam and the side wall of the cylindrical case.

CONSTITUTION: Side walls of a case body 1 and a case bottom plate 2a are folded and joined together by caulking, so that a cylindrical case 3a is formed. After a side plate is then fitted into one opening of the cylindrical case, a folded joint 6 is filled with an adhesive 4 with the side plate tilted downwardly. The adhesive 4 is hardened with the cylindrical case 3a upright, so that a shield layer, consisting of the adhesive 4, is formed inside the joint between the folded joint 6, the cylindrical case 3a, and the side wall. In such a fabrication technique for a waterproof ballast case, formed on the bottom plate 2a is a dam 8 which is in proximity to the folded joint 6, which is taller than the joint 6, and which is parallel with the side wall of the cylindrical case 3a, thereby constituting a channel 9 between the dam 8 and the side wall of the cylindrical case 3.