A length L between the markings 11a measured by the pre-coating thickness measuring device 17 and a length L′ between the markings 11a measured by the post-coating thickness measuring device 18 are calculated, and a length ratio is obtained. Measurement positions of thickness data measured by the post-coating thickness measuring device 18 are corrected based on the calculated length ratio. A value at each measurement position of pre-coating thickness information on a corrected post-coating thickness interpolation line connecting measurement points of corrected post-coating thickness information obtained by correcting the measurement positions with a line segment is determined as post-coating thickness at each measurement position. A difference between the determined value and the pre-coating thickness is calculated. If it is in a predetermined range, the electrode is determined as a non-defective product.