In the present invention, two coil-shaped probes each detecting the intensity of a magnetic field in a direction around a center axis of a processing space are provided in a process vessel of a plasma processing apparatus. Each of the probes detects an induced electromotive force generated in the coil, and a computer calculates an amount of radio-frequency current from the induced electromotive force, based on a predetermined calculation principle. A difference between the amounts of the radio-frequency current detected by the probes is calculated, and a loss radio-frequency current amount passing out of a plasma area between upper and lower electrodes is calculated, whereby the flow of the radio-frequency current in the plasma is known.