PURPOSE: To obtain a power tube member with sufficient mechanical strength by greatly reducing the volume of a recrystallized weakened area developing between linear members during spot welding by applying a voltage with a trapezoidal waveform (on a coordinate where the horizontal axis represents time and the vertical axis represents voltage) to a welding electrode.

CONSTITUTION: A voltage with a trapezoidal waveform is applied to a welding electrode for a very short time (ex., for 1ms) in order to prevent recrystallization of tungsten. Even when recrystallization of tungsten occurs during welding, a part 7a of recrystallized tungsten is discharged outside the welded area because the welding is completed in a very short time. Therefore it is possible to greatly reduce the volume of a recrystallized area 8 which is formed between linear members 6a and 6b by spot welding. When welding is completed in such a short time, thermal strain generally remains in the linear members 6a and 6b with high possibility. In this invention, however, since voltage increase and voltage decrease take place during welding, it is possible to fully avoid development of thermal strain.