PURPOSE: To enhance the peel strength of joints between thin metal plates by a method wherein a plurality of rectangular calks to fasten together first and second thin metal plates are arranged in a zigzag form.

CONSTITUTION: A heating chamber 10 is composed of a first thin metal plate 12 and a second thin metal plate 13 fastened together in such a calking manner that a plurality of rectangular calks 14 consist of longitudinal rectangular calks 15 lateral rectangular calks 16 arranged alternately in a row formed like a zigzag. For this reason, the joints made between the first thin metal plate 12 and the second thin metal plate 13 composing the heating chamber 10 become sufficiently strong against the stress taking place in any direction.