To solve problems that an electromagnetic field feeder and a plasma device using it become large in sizes as a whole and expensive in price.

A rectangular/cylindrical circularly polarized wave converter 13 is used that is composed of a first rectangular waveguide part 31 in which one end is connected to the other end of the rectangular waveguide, an opening 14A which is formed on the first side wall of the first rectangular waveguide part 31 and to which one end of the cylindrical waveguide is connected, second and third rectangular waveguide parts 32, 33 in which one-side ends are connected to the second side wall and the third side wall of the first rectangular waveguide part 31 and in which other ends are short-circuited electric-functionally, and a protrusion 34 which is formed in the tube of the first rectangular waveguide part 31, which is protruded from the inner face of a fourth side wall opposed to the first side wall toward the opening 14A, and which has conductivity at least on the surface.