PURPOSE: To make it possible to determine the degree of weld penetration with high reliability, by applying a thermo-paint on the rear side of a padded portion of a weld of a predetermined thickness before welding thereby to discriminate the color-changing state of the padded portion.

CONSTITUTION: A thermo-paint 3 is applied in a horizontally straight line at the rear side of a thick plate 1. The paint 3 employed here represents purple at normal temperatures, but changes to be write when it is heated up to 450°C or higher and never changes the color once it changes the original color. That is, the paint 3 is an irreversible thermosensitive paint. When the thick plate 1 after applied with the paint 3 is welded, sufficient calory necessary for penetration is transmitted to the rear side of the plate 1. And if the temperature is raised to 450°C or higher, the paint 3 on the reverse side of the plate changes its color. Accordingly, by measuring the width L of the color-changed portion of the plate, the depth of penetration of welding beads into the thick plate 1 can be guessed.