Guide members (37) extending from the microwave entrance to a ring member (34) are arranged in the direction of propagation of microwave in a radial waveguide. The guide members (37) contribute to prevention of complex electromagnetic mode due to a microwave reflected from the peripheral portion of the radial waveguide. Therefore, a uniform plasma can be produced because the radiation into the process chamber is uniform even not by disposing any electromagnetic absorbing member at the peripheral portion of the radial waveguide. Since the microwave reflected from the peripheral portion of the radial waveguide can be used to produce a plasma if any electromagnetic absorbing member is not disposed, the plasma can be produced efficiently, and excessive heat is not generated.