The present invention relates to a plasma processor capable of regulating the temperature of the inner surface of the processing chamber efficiently and with excellent response, with a low-cost configuration. A plasma processor 1 includes a processing chamber 11, a processing gas supply device 20, an exhaust device 40, coils 23, a high-frequency power supply unit 24, a heater 26, a cooling device 30, and a control device 50. The cooling device 30 is configured with a cooling member 32 facing the processing chamber 11 at a distance therefrom, a cooling fluid supply section 31 for supplying cooling fluid into a cooling passage 32a of the cooling member 32 and circulates it, and annular seal members 35 and 36 provided between the cooling member 32 and the processing chamber 11. The exhaust device 40 reduces the pressure in a space S surrounded by the seal members 35 and 36, the cooling member 32, and the processing chamber 11. The control device 50 controls the exhaust device 40 to reduce the pressure in the space S when high-frequency power is not applied to the coils 23, and to set the pressure in the space S at atmospheric pressure when high-frequency power is applied to the coils 23.