In certain embodiments a plasma is supplied from a plasma chamber 10 into a reaction chamber 18 of a plasma CVD apparatus. An electrode 22 is disposed in the reaction chamber 18. A semiconductor wafer on which a thin film is to be formed is placed on the electrode 22. A radio-frequency wave is generated by a radio-frequency wave generator 28 and supplied to the electrode 22 via a radio-frequency matching network 30, a blocking capacitor 32, and an RF probe 34 so as to control the plasma in the plasma chamber 10. A judgment device 38 is electrically connected to the RF probe 34. The voltage and current are be measured by the RF probe and the judgment device 38 is used to judge the state of the plasma in the plasma chamber 10.