PURPOSE: To prevent a banana shape caused by the irregular thickness of a product, by blanking a strip for forming plural caulking projections arranged concentrically at equal pitch and an iron-core piece to be laminated, and deviating the caulking recessed part of a ready-made iron core by the specific turning angle of a die to form a laminated core piled at caulking part.

CONSTITUTION: A projecting part 42 formed by blanking a strip 40 by means of a punch 51, and turning a die 2 by means of a chain 5 and a sprocket wheel 4, interlocked with a stroke of punch 51. In this case, the projecting part 42 is fitted to a recessed part 45 of a ready-made iron core 47 to well assemble them, by setting a turning angle θ to an equation, 360×(t)tanα/2π×R, where α: a skew angle of the side face of laminated iron core, R: a radius to the projecting part 42 of iron core, and (t): sheet thickness. Further, by turning the die 2 plural times, a banana shape, the warp of laminated core caused by the irregular thickness of strip 1, is prevented.