PURPOSE: To enable an upper electrode plate and a lower electrode plate dis placed from each other in position to be easily realigned with each other by a method wherein a sensor mounting hole where an active part or a passive part is mounted and a quartz cover are provided to the upper electrode plate and the lower electrode, respectively.

CONSTITUTION: An optical sensor 4 is mounted on each of four mounting hole 2a provided to the periphery of an upper electrode plate 2, and a light emitting diode 5a is mounted on each of four mounting holes 5a provided to the periphery of a lower electrode plate 5 corresponding to the mounting holes 2a. Quartz covers 3 and 6 are provided to the electrode plates 2 and 5 respectively so as to prevent the optical sensors 4 and the light emitting diodes 7 from being directly exposed to plasma. Before an etching operation is performed, if fixed parts get out of position due to deterioration, the fixed parts are repaired to align the upper electrode plate 2 with the lower electrode plate 5 so as to set all the four light emitting diodes 7 coincident with all the four optical sensors 4 in position.