The gap between upper and lower electrodes 1, 2 is measured at constant intervals of time while a contact 5 and a base 6 are pressed by the upper and lower electrodes under a constant pressure. Supply of an electric current to the upper and lower electrodes is controlled upon elapse of a predetermined interval of time after the gap has reached a maximum 18. The amount of metal melted out of a joining interface between the contact 5 and the base 6 is thus quantized to improve and stabilize the quality of joining between the contact 5 and the base 6.