Plasma processor

The invention relates to a plasma processor capable of correctly detecting arc discharge generation; the plasma processor using plasma to process a substrate (G) comprises the following structures: a chamber (11) supplied with high frequency power so as to generate plasmas in an inner side; a first differentiating circuit (21) used for carrying out time differentiation for travelling wave voltage Vf of the high frequency power; a second differentiating circuit (23) used for carrying out time differentiation for reflected wave voltage Vr of the high frequency power; a comparator (22) calculating dVr/dt-dVf/dt, when the calculated dVr/dt-dVf/dt exceeds the arc discharge detection value, the arc discharge is determined to happen in the chamber (11), so a signal is sent; and a diode, when the dVf/dt is a negative value, the dVf/dt is set to be 0, so the dVr/dt-dVf/dt can be decreased.