Plasma measuring method and plasma processing apparatus

A plasma processing apparatus including a measuring circuit including a signal transmitter that outputs an AC voltage, and a controller including a communication part and a control part,

• wherein the communication part is configured to:
• receive a first current measured by the measuring circuit when the AC voltage is outputted from the signal transmitter to a probe device in a state where plasma is not generated in the plasma processing device;
• and
• receive a second current measured by the measuring circuit when the AC voltage is outputted from the signal transmitter to the probe device in a state where plasma is generated in the plasma processing apparatus;
• the control part is configured to:
• measure a phase difference between a current flowing through the plasma and the AC voltage by vector calculation using the received first current and second current;
• and
• adjust the AC voltage based on the phase difference such that a voltage applied to the plasma becomes constant, and measure a plasma state based on the current flowing through the plasma,
• wherein, based on that the current flowing through the plasma and the voltage applied to the plasma have a same phase, the AC voltage is adjusted by using the phase difference as a phase difference between the voltage applied to the plasma and the AC voltage.