A plasma formation region control apparatus for controlling a region of plasma formation under a pressure equal to or exceeding atmospheric pressure, the plasma formation region control apparatus comprising: one or a plurality of electromagnetic wave oscillators; one or a plurality of antennas (118; 318) connected to the electromagnetic wave oscillator; and control means (108, 112; 108, 212) for controlling the electromagnetic wave oscillator and the antenna (118; 318), wherein the control means (112; 212) is configured to: establish a sequence for driving the electromagnetic wave oscillator based on: (a) the state of radicals, ions, or a gas, or the electron density, within a specified plasma; or (b) a state quantity of the specified plasma, the intensity of plasma emission, or the intensity of an emission element within a predetermined wavelength range, or of the electron temperature, travel velocity, or path; and drive the electromagnetic wave oscillator according to the sequence, characterised in that: the plasma formation region control apparatus comprises a spark plug; the control means (108, 112; 108, 212) is configured to generate a control signal for indicating the timing of a discharge produced by the spark plug; and the control means (112; 212) is configured to establish the sequence so that expansion caused by the emission of electromagnetic waves ceases when a time period that is shorter than a thermal relaxation time of electrons and radicals, ions, or gas generated in the plasma formation region elapses from a scheduled time of plasma generation so as to obtain an expanded non-equilibrium plasma only. one or a plurality of electromagnetic wave oscillators; one or a plurality of antennas (118; 318) connected to the electromagnetic wave oscillator; and control means (108, 112; 108, 212) for controlling the electromagnetic wave oscillator and the antenna (118; 318), establish a sequence for driving the electromagnetic wave oscillator based on: (a) the state of radicals, ions, or a gas, or the electron density, within a specified plasma; or (b) a state quantity of the specified plasma, the intensity of plasma emission, or the intensity of an emission element within a predetermined wavelength range, or of the electron temperature, travel velocity, or path; and (a) the state of radicals, ions, or a gas, or the electron density, within a specified plasma; or (b) a state quantity of the specified plasma, the intensity of plasma emission, or the intensity of an emission element within a predetermined wavelength range, or of the electron temperature, travel velocity, or path; and drive the electromagnetic wave oscillator according to the sequence, characterised in that: the plasma formation region control apparatus comprises a spark plug; the control means (108, 112; 108, 212) is configured to generate a control signal for indicating the timing of a discharge produced by the spark plug; and the control means (112; 212) is configured to establish the sequence so that expansion caused by the emission of electromagnetic waves ceases when a time period that is shorter than a thermal relaxation time of electrons and radicals, ions, or gas generated in the plasma formation region elapses from a scheduled time of plasma generation so as to obtain an expanded non-equilibrium plasma only. the plasma formation region control apparatus comprises a spark plug; the control means (108, 112; 108, 212) is configured to generate a control signal for indicating the timing of a discharge produced by the spark plug; and the control means (112; 212) is configured to establish the sequence so that expansion caused by the emission of electromagnetic waves ceases when a time period that is shorter than a thermal relaxation time of electrons and radicals, ions, or gas generated in the plasma formation region elapses from a scheduled time of plasma generation so as to obtain an expanded non-equilibrium plasma only.