An antenna apparatus includes a slit provided on an antenna element between first and second feed ports; and a series resonant circuit provided at a location along the slit, with a distance from an opening of the slit. When the antenna apparatus operates at a first isolation frequency identical to a resonance frequency of the series resonant circuit, the series resonant circuit is short-circuited, and only a section of the slit from its opening to the series resonant circuit resonates, thus providing isolation between the feed ports at the first isolation frequency. When the antenna apparatus operates at a second isolation frequency lower than the first isolation frequency, the series resonant circuit is open, and the entire slit resonates, thus providing isolation between the feed ports at the second isolation frequency.