An antenna device includes a plurality of antenna elements respectively configured to receive incident waves coming from an object, a modulating unit respectively configured to modulates a first received signal of the incident waves output from the antenna elements into a second received signal, the second received signals having a plurality of different frequencies and phases corresponding to polarization directions of the received incident waves, a synthesizing unit configured to synthesize the plurality of second received signals and generates a synthetic signal, a signal processing unit, configured to perform predetermined signal processing on the synthetic signal, and an extracting unit configured to extract third received signals for each frequency and each phase from the synthetic signal on which has been performed the predetermined signal processing.