To suppress damages of a punch in forming an engagement joining part by providing a relief surface on an edge part of the punch forming the engagement joining part on a strip to expand an edge surface, to prevent high concentration of the pressing load, and to distribute the load over an extensive area.

In a band stretching device 5 where a strip is wound in a condition where the tension is applied to the periphery of a packaged body and two end parts of the strip are joined with each other by a plurality of engagement joining parts, a relief surface having short and long edges 55', 57' is formed on a corner part C' of a punch 50 to be used together with a die to form a slit of the engagement joining parts. The excessive pressing force is not concentrated only along the edge outside the punch 50, and is distributed over a remarkably extensive area, and the cutting of the slit to the strip is started toward an inner corner part D' on which a strong and wide edge surface of the punch 50 is provided. As a result, a problem of damages generated in the pressing process can be solved.