PURPOSE: To discriminate exactly whether a seam weld zone is defective or non-defective by cutting a part of the seam weld zone in the direction orthogonal with a weld line, sampling a test specimen from the section thereof, etching the section of the specimen and evaluating the metal flow of the weld zone.

CONSTITUTION: A sectional test specimen in the direction A orthogonal with the weld line L of a welded material 1 obtd. by welding the same materials under the same condition as in the actual operation and the section of said specimen is etched then the metal flow of the weld zone is evaluated. Whether the weld zone is defective or non-defective is thus exactly discriminated.