PURPOSE: To enable the exact observation of welding phenomena without fluctuation in actual welding by providing a notched part on the side opposite from a welding surface on one side face of a base material and generating an arc along the contact surface between the quartz glass fitted in the notch and the side face of the notch.

CONSTITUTION: A notched part 11 is provided on one side face of a base material 1 on the side opposite to the welding surface 10 of the material 1 except a flange-shaped part 12 which is allowed to remain on the face 10 side. The notched part 11 extends along the one side face of the base material. The one end edge of quartz glass 2 is tightly fitted in the notched part 11. The glass 2 is then disposed with its observation surface exposed to the outside without intervening in the welding surface of the base material 1. A welding wire 4 is positioned along the contact surface between the glass 2 and the side face of the notched part 11, and an arc 3 is generated along the contact surface. On the other hand, a light source 6 for extinguishing the arc is disposed so as to face to the welding position. The welding is observed with a camera 5 while it is progressed in an arrow direction.