A plasma processing apparatus can control a ratio between an input power during a pulse-on period and an input power during a pulse-off period by a matching operation of a matching device provided on a high frequency transmission line for supplying the high frequency power as a continuous wave without a power modulation. An impedance sensor 96A provided in a matching device of a plasma generation system includes a RF voltage detector 100; a voltage-detection-signal generating circuit 102; an arithmetic-average-value calculating circuit 104; a weighted-average-value calculating circuit 106; and a moving-average-value calculating unit 108 of a voltage sensor system, and also includes a RF electric current detector 110; an electric current-detection-signal generating circuit 112; an arithmetic-average-value calculating circuit 114; a weighted-average-value calculating circuit 116; a moving-average-value calculating unit 118; and an impedance calculating circuit 120 of an electric current sensor system.