Provided is an antenna which has a small-sized planar constitution as can be easily mounted on a small-sized radio device and which can form a principal beam having a vertical polarization in directions of low and high elevation angles.On the surface of a substrate 11, slot elements 13A and 13B having a length of about one half wavelength are arranged in parallel at a predetermined distance d1, and a reflecting plate 14 is arranged at a predetermined distance h from the mounting face of the slot elements 13A and 13B. On the back of the substrate 11, parasitic elements 15A to 15D are made of a copper foil pattern and are arrayed to intersect the slot elements 13A and 13B at right angles. A switching element 16A is connected with the parasitic elements 15A and 15B, and a switching element 16B is connected with the parasitic elements 15C and 15D.