Abstract: An upper shearing blade 3 equipped with a protrusion 30 of a triangle-columnar shape and a lower shearing blade 4 equipped with a protrusion 40 of the same shape are applied onto the overlapped portion of metal plates 1 and 2 to be bonded, and then pressed into the metal plates 1 and 2 in an oblique direction inclined with respect to the thickness direction by a stroke in such a range that the metal plates 1 and 2 are not completely cut off. The operating loci of the upper shearing blade 3 and the lower shearing blade 4 are overlapped each other so that one falls inside the other, and the sheared surfaces of the metal plates 1 and 2 are formed into a bonded portion by plastic flow deformation. Therein, since a compressive force is applied onto the portions to be bonded, the portion being defined by the amount of overlap, and the portions are compressed to form a compressed portion after completion of bonding, the bonding strength enhances.