PURPOSE: To attain storage battery casings having enough rigidity for large type or superlarge type by a method wherein resin with electrolyte resistance performance is injected into a mold through first gates in communication with through holes of a core material and second gates other than those first ones.

CONSTITUTION: Resin 12 residing in a runner 7 is injected from first and second gates 10, 11 through first and second spurs 8, 9. At this time, the resin 12 injected from the first gates 10 is quickly pushed into an inner cavity 19 via through holes 14 to fill the same. Meanwhile, the resin 12 injected from the second gates 11 is quickly pushed into an outer cavity 20 to fill the same. The resin spreads over the entire cavity between a male mold 4 and a female mold 5 including both inner and outer cavities 19, 20. After curing of the resin 12, the male mold 4 is removed from the female mold 5 and then the molding product is taken out to obtain a storage battery.