| 摘要 |
A noble metal chip 31' is superimposed on a surface of a central electrode 3 made of a heat resisting alloy, the main component of which is Ni or Fe, to which the chip is secured so that a superimposed assembly 70 is constituted. A perimeter laser weld portion 10 across the noble metal chip 31' and a chip securing surface forming portion is formed around the outer surface of the chip of the superimposed assembly 70. Thus, the noble metal chip 31' is secured to the chip securing surface so that a noble-metal igniting portion 31 is constituted. To form the perimeter laser weld portion 10 which has a maximum outer dimension dmax which is smaller than 2.0 mm and which does not reach the discharging surface 31a in a direction of the thickness of the noble metal chip 31', a laser beam source 50 is, as a light source of laser beams for use in a welding operation, employed-which is arranged such that energy per pulse is 2 J to 6 J, the length of the pulse is 1 millisecond to 10 milliseconds and the pulse generating frequency is 2 pulse/second to 20 pulse/second.
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