PURPOSE: To obtain an averaged temp. on the working surface of a heating chip by attaching plural pieces of thermocouples to the chip and by stopping the supply of electric power, detaching the chip from terminals and again repeating the previous operation when the temp. difference detected with each thermocouple is above the set value.

CONSTITUTION: The thermocouples 11W13 are attached to the plural points of the chip 10 and the chip 10 is pressed onto the terminals 15 of multi-terminal parts to be joined to a printed circuit board 14. The current is supplied to the chip to heat the same when the prescribed pressure is attained. The conduction of the current to the chip 10 is stopped and after the chip 10 is once detached from the terminals to be connected, the previous operation is again repeated when the temp. differences of the thermocouples 11W13 are above the set values. The multiple terminals of the flat lead parts are thereby simultaneously and surely joined.