PURPOSE: To treat a material to be treated with plasma at a high treating rate without damaging the material by supplying a raw gas into an activation chamber consisting of a microwave-transmissible material, and introducing the activated gas into a reaction chamber through a transport part impressed with a magnetic field.

CONSTITUTION: A microwave from a microwave source 7 is injected into a cavity resonator 2 provided with an adjusting plate 8 from a hole 4 through a tuner 6 to form a standing wave. The activation chamber 1 consisting of a microwave- transmissible material such as quartz is arranged in the cavity resonator 2, and plasma is generated by the microwave penetrating from the whole wall surface to highly activate the raw gas introduced from a supply pipe 9. The activated gas is transported into the reaction chamber 15, which is exhausted from an exhaust pipe 17, from the activation chamber 1 through a transport pipe 11. A couple of magnets 12a and 12b provided with a yoke 13 are provided to the transport pipe 11 to impress a magnetic field. The charged particles and ions in the activated gas are trapped or deflected by the line of magnetic force 14 orthogonal to the axis of the transport pipe 11. The material 18 to be treated in the reaction chamber 15 is treated with the highly activated gas, freed of impurities.