An antenna device has a circular radiation plate with a diameter of substantially wavelength in electric length faced to a ground plate with a first power supply port and a second power supply port in its periphery. The power supply ports are disposed at positions where straight lines connecting respective power supply ports to a midpoint of the radiation plate intersect at right angles. Four slits axisymmetric with respect the straight lines are disposed in radiation plate, and two sides of the periphery of each slit contact with each other.