A method prevents a cylindrical body from buckling in externally curling edges of both side portions by pressing curling dies in an axial direction after molding the cylindrical body having a drop portion whose outer diameter is small and two side portions whose outer diameter is large by enlarging a cylindrical material made of aluminum alloy by electromagnetic molding. In curling the edges of the side portions, a buckling preventing die restrains an outer periphery of the drop portion of the cylindrical body. Because the buckling preventing die receives the axial compression force of the curling dies, it can prevent the cylindrical body from buckling and deforming. The buckling preventing die is what a split die disposed on the side of an outer periphery of the cylindrical material in the electromagnetic molding is used as.