According to the plasma treatment on an object accommodated in the processing room, the plasma treatment is carried out as follows. The discharge detecting sensor detects a signal of potential change caused with change in plasma discharge. Receiving the signal, the signal recording section temporarily records the signal as signal data indicating potential change. Referencing the signal data, the signal analysis section extracts index data. The index data shows a condition of plasma discharge, for example, as a count value for discharge-start waves, a count value for abnormal discharge, a count value for feeble arc discharge. The device control section judges a condition of plasma discharge by monitoring the index data and carries out the retry process, the accumulative plasma process, and the maintenance judgment process for performing plasma treatment operations properly.