PURPOSE: To always warrant the quality of a weld zone by detecting an isothermal line from a detected surface temperature distribution of materials to be welded, calculating the area surrounded thereby, comparing the area with the reference area and stopping a welding current applied to the materials to be welded when both are coincident with each other.

CONSTITUTION: The surface temperature distribution of the material 2a to be welded corresponding to the peripheral parts of welding electrodes 1a and 1b is detected by an optical image fiber 4 and a temperature detector 5. The isothermal line is detected from the detected surface temperature distribution and the area surrounded thereby is calculated by the temperature detector 5 and a temperature signal processing circuit 6. The area is compared by a signal comparator 11 with the reference area and when both areas are coincident with each other, the welding current applied to the materials 2a and 2b to be welded from the welding electrodes 1a and 1b is stopped by a control means. Consequently, the controller can be miniaturized.