A surface-layer conductive plate having an opening is disposed on a first surface of a dielectric substrate. A radiation electrode is disposed inside the opening on the first surface of the dielectric substrate. A ground conductive plate is disposed on a second surface of the dielectric substrate, the second surface being opposite to the first surface. Interlayer connection members are disposed so as to surround the opening as seen in a plan view. The interlayer connection members electrically connects the surface-layer conductive plate to the ground conductive plate and defines a cavity that causes electromagnetic resonance to occur. A reactance element is configured to cause an impedance that a side face of the cavity exhibits with respect to an electromagnetic wave propagating in the cavity to include a reactance component.