PURPOSE: To join firmly and easily two pieces of metallic sheets together by stretching and breaking metallic strips between two parallel notched lines provided to the surface of overlapped two metallic sheets and overlapping the front ends of two formed metallic pieces with each other to press them.

CONSTITUTION: Two parallel notched lines 3, 3' are provided to overlapped two metallic sheets 2, 2' by a knife 1 with sharp edge. Thereafter, the knife 1 is further lowered till the central part of a metallic strip 4 between the notched lines 3, 3' breaks. Next the two metallic pieces 4a, 4b formed by said breakage are pressed so that their front ends overlap with each other. Then the two metallic sheets 2, 2' are caulked and joined together.