The electrical length of the parasitic antenna of a pair of 2-element Yagi antennae is varied to vary the directivity of the antenna. A radio device (12) outputs a receiving signal corresponding to the received electric field strength of a first antenna (10), a controller (15) outputs a control signal corresponding to the result of detection of the received signal to a variable impedance circuit (14), and the impedance of the variable impedance circuit (14) is varied in accordance with the control signal. The electrical length of a second antenna (13) which is parallel with the first antenna (10) with a certain spacing is varied and hence the second antenna (13) serves as a waveguide for a reflector to vary the directivity of the antenna.