To diagnose the wearing state of an electrode in on-line, to secure the safety of a working, to save maintenance cost and man hours by using a two-dimensional range finder and directly measuring the cross sectional profile of the butting surface of the electrode three-dimensionally.

The discharging surfaces at the upper and the lower sides of the output side electrodes 3a, 3b and the input side electrodes 4a, 4b are made to butting state, and gap between the butting surfaces is measured by the two-dimensional range finder 10 to obtain two-dimensional data at this position. Then, a carriage 12 is moved in the plate width direction successively, and while traversing the two-dimensional range finder 10, the whole width of the electrode is measured. By this method, the cross sectional profile of the butting surface of the output side electrodes 3a, 3b can be obtd. three-dimensionally. Successively, a rotary table 14 is turned at 180° angle, and the gap between the butting surfaces of the input side electrodes 4a, 4b in the state of butting the discharging surfaces at the upper and the lower parts is measured, and while gradually pulling back the carriage 12, the whole width of electrode is measured similarily. By this method, the cross sectional profiles of the butting surfaces of both electrodes can be obtd. three-dimensionally.