Abstract: A plurality of concentric ring-shaped slots (300) to (304) are formed in a planar antenna member (3), and the thickness of conductors in the central part is made relatively thin and the thickness of peripheral conductors is made relatively thick, so that a microwave can easily pass through the slots (300) to (304) without being attenuated, and a uniform electric field distribution can be provided and uniform high-density plasma can be generated in a processing space on an average. As a result, an object to be processed can be provided close to the antenna member (3) and the object can be uniformly processed at high speed.