1. A method for controlling an array antenna, said array antenna comprising: a radiating element for receiving a radio signal; a plurality of parasitic elements provided apart from said radiating element by a predetermined distance; a plurality of variable-reactance elements connected to said plurality of parasitic elements, respectively; and controlling means for changing a directivity characteristic of said array antenna by changing each reactance value set to each of said variable-reactance elements so that each of said parasitic elements operates as either one of a director and a reflector, wherein said method includes a step of iterating the following steps of: upon setting the reactance values of said respective variable-reactance elements by randomly perturbing the reactance values from predetermined initial values, calculating predetermined cross correlation coefficients between a received signal and a training sequence signal before and after the perturbation, the received signal being obtained by receiving by said array antenna a training sequence signal contained in a radio signal transmitted from a remote transmitter, and the training sequence signal being generated so as to have a signal pattern identical to that of the transmitted training sequence signal; selecting and setting reactance values when the cross correlation coefficient increases between those before and after the perturbation; and setting reactance values obtained by randomly perturbing the selected reactance values, to the variable-reactance elements, respectively.