An antenna device provides two power feed ports provided, respectively, at the specified positions on an antenna element (1). The antenna element (1) is simultaneously excited though each power feed port in order to simultaneously operate as two antennas corresponding to each of the two power feed ports. The antenna device is positioned between the two power feed ports and is provided with a slit(S1) which varies the resonance frequency of the antenna element (1) and produces the specified isolation between the power feed ports at the specified isolation frequency and rectification means (11,12) which shift the operating frequency of the antenna element (1) from the changed resonance frequency to the isolation frequency.