An impedance matching device is provided with a basic element having variable characteristic parameters for impedance matching, and an auxiliary element having variable characteristic parameters. At the time of generating plasma by using the impedance matching device, the characteristic parameters of the basic element of each antenna element are fixed, respectively, and the characteristic parameters of the auxiliary element are adjusted for each antenna element. Thus, in an adjusted status where impedance matching for each antenna element is adjusted, each antenna element of an antenna array is fed with a high frequency signal, an electromagnetic wave is radiated from the antenna element, the characteristic parameters of the basic element of each antenna element are synchronized and adjusted, and the impedances of the whole antenna array are matched.