An antenna device for operating in a predetermined frequency band has a resonator section, a semiconductor section and an antenna section. The resonator section includes a first conductor section, a dielectric section, and a second conductor section for specifying a reference potential against each section which is arranged so as to oppose the first conductor section through the dielectric section. A semiconductor section is arranged so as to be sandwiched between the first conductor section and the second conductor section. The antenna section uses the second conductor section as a grounding conductor, is substantially spherical, makes at least its surface electroconductive, and is arranged on the first conductor section.