A small and light helical antenna, with a high gain, includes arranging the dielectric cylinder with a helix conductor on the conductive plate and arranging a thin rectangular conductive plate between the lower end of the helix conductor and the feed point in a predetermined position of the conductive plate. The impedance matching is between the input impedance of the helix conductor and a power feeder. The upper and lower conductive plates constitute parallel plate waveguide space, and the probe which comes into the parallel plate waveguide space is in the lower conductive plate side, and the lower end of a thin rectangular conductive plate is connected with the lower end of a probe.