Disclosed is an antenna device in which an excitation phase changing unit (4) for changing the excitation phases of element antennas (2-1n) to (2-8n) by controlling phase shifters (3-1n) to (3-8n) dependently on preset phase values is disposed, and every time the excitation phases are changed by the excitation phase changing unit (4), a radiation pattern forming unit (12) shifts the phases of a sum signal and a difference signal, and forms a monopulse sum pattern by time-integrating the sum signal after phase shift and also forms a monopulse difference pattern by time-integrating the difference signal after phase shift.