To provide a compact antenna device having good isolation characteristics.

The antenna device comprises: a conductor part 3 on which a conductive pattern consisting of remaining segments formed by removal of a plurality of segments into which a rectangular conductive plate is divided is formed; and a feeding part 4 provided on a corner of the conductor 3. The antenna device also comprises two planar antenna elements 2 for transmitting/receiving rightward circularly polarized waves on a surface and leftward circularly polarized waves on a rear surface. The two planar antenna elements 2 are arranged in a horizontal direction on a same plane, and the left side planar antenna element 2 is arranged so that its surface S is oriented to a front surface side and the feeding part 4 is positioned at the lower left, and the right side planar antenna element 2 is arranged so that its rear surface R is oriented to a front surface side and the feeding part 4 is positioned in the lower right.