The invention provides a method for detecting and managing the status of a plasma processing apparatus with high sensitivity so as to enable long-term stable processing. In a plasma processing apparatus comprising a vacuum processing chamber 10, a plasma generating high frequency power supply 16, and a measurement device unit 3 for estimating the status of the apparatus via reflected waves 54 of the incident waves 53 reflected from the processing apparatus including a waveform generator 32, a VCO 33, a directional coupler 34, a detector 35 and a measurement data processing unit 36, frequency-swept high frequency waves 53 for measurement are introduced to the processing chamber where no plasma discharge is performed, so as to monitor the change of absorption spectrum frequency of the reflected waves 54 to thereby monitor the change in status of the processing apparatus.