To uniformize surface treatment by preventing distortion deformation due to thermal expansion in a slot plate, arranged at the lower face side of a dielectric plate in the antenna guide of a radial line slot antenna, in which plasma by microwaves is generated with the surface of a semiconductor substrate when conducting the surface treatment of the semiconductor substrate.

The inside part of at least the mounting part of a slot plate 7 on an antenna guide 2 is provided with an expansion absorbing part 11, constituted by perforating a through-hole 11b, so that a deformable fine width piece 11a for connecting the further outside part with the inside part can be left so that the expansion absorbing part 11 can be circularly extended along the whole periphery. Thus, thermal expansion at the inside part is prevented from exerting influence on the outside part.