PURPOSE: To maintain electric discharge of high stableness by adjusting the matching condition of a microwave power supply system, lessening fluctuation of the light emission intensity, and measuring the light emission intensity of plasma.

CONSTITUTION: The light emission intensity of plasma 16 is measured by an intensity measuring mechanism 14 by taking out the emitted light from a takeout port 13 in a microwave lead-in part 11 through an optical cable 17. The locations of mounting sensor (s) and/or optical fiber 17 to measure the intensity are either outside or inside a vacuum vessel 1. The information on the intensity of the plasma 16 in the vessel 1 obtained by the mechanism 14 is sent to a tuner control mechanism 12 and used in adjustment control of a tuner 9, and the matching condition of a microwave waveguide path 10 in the microwave power supply system is adjusted and controlled. The matching condition is adjusted by control means 12, 12a by the mechanism 14 and on the basis of output signal therefrom, and fluctuation of the intensity is lessened.