PURPOSE: To surely and simply identify and verify a recessed defect caused by the phase change of a plated film owing to heating by dripping high- concentration strong acid at the portion of the plated film.

CONSTITUTION: Nitric acid having the concentration of about 10 normal is dripped at the portion where the crystallization of electroless nickel-phosphorus(NiP) is to be confirmed, and the nitric acid is washed away with water several sec later. The inherent dissolution surface state such as whether a plated film is dissolved or not and a chrysanthemum flower shape is observed to verify whether the plated film is crystallized or there is another surface defect. The defective plated film portion owing to high-temperature heating is crystallized. When nitric acid is dripped at this recessed defect section, amorphous NiP is not dissolved, and only crystallized nickel phosphide such as Ni3P is dissolved. The crystallization section locally generated in an amorphous material is observed as the state recessed from the periphery (fine recessed surface defect: bit). No special facility is required except for a microscope, and this verification can be easily and simply conducted.