An object is to provide a plasma processing device capable of highly accurately monitoring an operation state including whether or not the plasma discharge is executed, whether the discharge is normal or abnormal and whether or not the maintenance work of the vacuum chamber is necessary.A discharge detection sensor 23, in which a dielectric member 21 and a probe electrode unit 22 are combined with each other, is attached to an opening portion 2a provided in a lid portion 2 composing a vacuum chamber. A change in electric potential induced according to a change in plasma discharge in a probe electrode is received by a plurality of wave-form detecting portions and a detection signal is outputted each time a change in electric potential agreeing with a predetermined different condition appears. The detection signal outputted from the corresponding wave-form detecting portion is counted by the plurality of wave-form detecting portions and the counted value is held. According to the counted value, an operation state is highly accurately monitored which includes whether or not the plasma discharge is executed, whether the discharge is normal or abnormal and whether or not the maintenance work of the vacuum chamber is necessary.