In a microwave plasma processing apparatus that uses a radial line slot antenna, a slot plate (16) is formed by a material having a thermal expansion rate close to the wave retardation plate (18), or depositing a metal on a dielectric plate constituting the wave retardation plate (18). An intimate contact between the wave retardation plate and a slot plate constituting a microwave radiation surface is improved so as to prevent an abnormal electric discharge. <IMAGE>
