An antenna apparatus 1 has an antenna substrate 21 composed of a separator 23 and electrolyte layers 24a and 24b disposed on both surfaces of the separator 23; an antenna pattern 22a disposed on the solid electrolyte layer 24a; and an antenna pattern 22b disposed on the solid electrolyte layer 24b. The antenna patterns 22a and 22b are made of an electroconductive plastic. When a DC voltage is applied between the antenna patterns 22a and 22b, ions can be doped to one of the antenna patterns 22a and 22b, whereas ions can be undoped form the other of the antenna patterns 22a and 22b. In other words, one of the antenna patterns 22a and 22b can become a conductor, whereas the other thereof can become an insulator.