A surface of copper foil wound onto a guide roller 26 is irradiated with light. Specular light from the copper foil surface is received by CCD cameras 14a, and scattered light from the copper foil surface is received by CCD cameras 14b. When, in a region in which the amount of specular light received by the CCD cameras 14a is equal to or larger than a first threshold, a portion having a luminance equal to or larger than a second threshold is present, and when the amount of scattered light received by the CCD cameras 14b is smaller than a luminance average, that region is determined to be a defective copper portion.