Many companies have been attempting to transfer the skills and knowledge of skilled workers to other inexperienced workers. The establishment of an efficient training method is also desired for gas tungsten arc welding (GTAW), which has high demand in the production fields of industrial equipment. This study focused on the brightness of the backside weld pool changing under GTAW conditions. The backside weld pool images during welding of a stainless steel plate were taken with a charge coupled device (CCD) camera from the bottom of the welding stage made of glass. The situation of the weld pool at the backside of the plate was visualized by a brightness map of the backside weld pool with the brightness on the horizontal axis and the number of pixels on the vertical axis. From this map, the torch control technique and quality of the backside bead could be numerically evaluated. Therefore, the map suggests that the GTAW skills could be visualized. This could improve welding skill easily and also contribute to highly efficient training, education, and transfer of skills, because the brightness map of the backside weld pool reveals the weaknesses of the welder.