PURPOSE: To evaluate the quality of brazing immediately, i.e. in real time by measuring the thermoelectromotive force between materials to be welded and evaluating the quality with this thermoelectromotive or thermoelectromotive force and a heating time.

CONSTITUTION: A probe 15 abuts elastically on a conductor 3 with the spring force of a spring 16 and another conductor 3 contacts a probe 17. A voltage detecting circuit 21 detects the thermoelectromotive force between conductors 2 and 3 which are detected by probes 15 and 17. When the thermoelectromotive force detected by a voltage detecting circuit 21 is not within a predetermined normal range, a stopping means 22 stops subsequent manufacturing processes and a display means 23 such as a buzzer and a red lamp indicates that the quantity of brazing is defective. When it is judges that the electromotive force detected by the voltage detecting circuit 21 is normal, a means 24 for working operation in the next process is activated.