Abstract: A first layer and a second layer are stacked one another and respectively have different dielectric constants. The dielectric constants are set, such that a first reflected wave and a second reflected wave have a same intensity. A thickness of the first layer is set, such that the first reflected wave and the second reflected wave have a phase difference to at least partially cancel each other. The first reflected wave is reflected on an exposed surface of the first layer when a target radio wave is incident on the exposed surface. The second reflected wave is a reflected wave that is incident from the exposed surface, reflected on a boundary surface between the first layer and the second layer, and emitted from the exposed surface. The exposed surface is on an opposite side of the boundary surface.