The magnitude (charge quantity) and energy of discharge are obtained by optical measurement based on light emission, and are evaluated. A discharge source is caused to emit discharge light by applying a voltage to the discharge source from a known power supply, the intensity waveform of the discharge light emission is measured using a light receiving element, the waveform of discharge current is simultaneously measured using a current conversion probe or a current waveform detector, and a database is created in which a relation with analysis data sets obtained by analyzing the waveforms is recorded in consideration of applied power information. The intensity waveform of discharge light emission from a piece of equipment under measurement is measured using the light receiving element, and light emission data obtained by analyzing the waveform is compared with the data recorded in the database so as to estimate the magnitude of discharge as a value.