PURPOSE: To raise the strength of a recessed for installing a detaining pin of a case, and to form it easily and economically, by bringing said recessed part to heat forming by utilizing a resistance heat generating means.

CONSTITUTION: The lower part of a case 2 is inserted into a lower electrode 8 and is camped by a cylinder in a state that the case 2 is supported by a case receiver 7, it is opposed up and down against an upper electrode 9, the case 2 is pressed between the electrodes 8, 9, and a recessed part 5 is subjected to heat forming. As for a minute contact area of the upper electrode 9 and the case 2, resistance heat is generated by a current from the upper electrode 9, and the outside circumferential face which is in contact with the upper electrode 9 of the case 2 is converted locally to high temperature, and is brought easily to heat forming by moderate pressure between both the electrodes 8, 9.