PURPOSE: To make it possible to easily detect a defect even when the defect involves a large amount of specular reflection components, by a method wherein, above an aluminum plate, such spot-like light sources that they are reflected in the surface of the aluminum plate are arranged in a lattice shape.

CONSTITUTION: An aluminum plate 7 having roll streaks 9 shows specular reflection characteristics in a direction parallel to the roll streaks 9 and diffuse reflection characteristics in a direction orthogonal to the roll streaks 9. Spot-like light sources 10 are arranged in a lattice pattern above the aluminum plate 7. As a result, striped light and dark patterns are formed on the surface of the aluminum plate 7 in a direction orthogonal to the roll streaks 9. If a defect 12 is present on the surface of the aluminum plate 7 at positions 11d, 11e where the dark pattern is reflected, the illuminating light L1 from the light source 10d shows reflection characteristics in which specular reflection components are large in amount at the damage 12, so that the damage 12 is brightly seen from an observation position 8. In this case, if the mounting height of the light source 10 is represented by H, the mounting distance La is set at 0.04H or less, while the mounting distance Lb is set at 0.8H or less, whereby it is possible to easily detect the defect.