PURPOSE: To increase the strength in the joint part of two sheets of roughly rolled plate materials and to make joining operation simple without using auxiliary members and materials by cutting the ends to be joined of said materials into mutually nested shapes superposing the ends and engaging compressively the ends on the same plane.

CONSTITUTION: The ends to be joined of two sheets of roughly rolled plate materials 1a, 1b are cut into mutually nested shapes 3. Such ends 3 are superposed above and below and are compressively engaged with each other. The strength in the joint part is increased by the nested type engagement and the ends are united to a uniform structure by rolling in the next stage. The need for auxiliary members and materials as in the conventional mechanical joining and welding is eliminated and the joining operation is simplified.