PURPOSE: To generate a plasma with high efficiency without using an electric discharge by bringing a combustion flame of a hydrocarbon into contact with a flame ionizing member prepared by sintering in an oxidizing atmosphere a molded composition comprising a substance which undergoes a variation in resistivity when irradiated with radiations and a magnetic substance, and applying a magnetic field to the flame in contact with the flame ionizing member.

CONSTITUTION: A composition prepared by mixing a substance which undergoes a variation in resistivity when irradiated with radiations with a magnetic substance is sintered in an oxidizing atmosphere. The mixing ratio, by weight, of the magnetic substance to the optically active substance is 5-40%, preferably 8-30%. The composition is ordinarily mixed with a binder, and the resultant composition is molded into a rod-like, spherical or other desired shape, followed by sintering. The sintering of the molded composition is carried out in an oxidizing atmosphere in an electric field at a temperature of 1,500°C or higher, preferably 1800-2000°C, to obtain a flame ionizing member. A combustion flame of a hydrocarbon is brought into contact with the flame ionizing member. The flame is ionized into a plasma state, which comes to extinction in a comparatively short time with the result of return to a normal neutral flame. For making the plasma state stable for a longer time, it is effective to apply a magnetic field to the combustion flame. Thus, sustained heating can be realized.