Abstract: A discharge tube (11) made of a dielectric material extends through a hole (3) of a rectangular waveguide (1) and through a coaxial microwave cavity (4) so as to be coaxial with the central axis of the cavity (4). This discharge tube (11) has a double-tube structure including an outer tube (12) and an inner tube (13). The sectional area of an annular gap formed between the outer tube (12) and the inner tube (13) is held constant over the entire length of the inner tube (13). This allows the generation of a stable thermal plasma when a reaction gas containing an organic halide and water vapor is supplied into the outer tube through the annular gap with a microwave transmitted from the rectangular waveguide (1) into the cavity (4).