An adaptive controller for an ESPAR antenna randomly perturbs a bias voltage vector V(n) composed of elements of bias voltage values Vm by a random vector R(n) generated by a random number generator, compares an objective function value J(n) of a cross correlation coefficient for a bias voltage vector V(n) before the perturbation with an objective function value J(n+1) of a cross correlation coefficient for a bias voltage vector V(n+1) after the perturbation, and selects and sets the bias voltage Vm corresponding to that when the cross correlation coefficient increases before and after the perturbation. Then the adaptive controller repeats the random perturbation and setting from the bias voltage of respective varactor diodes. This leads to that it is not necessary to provide a long training sequence signal, and the control process can be executed with learning so that a performance can be improved every iteration for search.