A variable resonator has a dielectric substrate 2, an input/output line 3 formed on the dielectric substrate 2, a first resonator 4 that has one end connected to the input/output line 3 and the other end grounded, and a second resonator that has one end connected to the input/output line 3 at the point of connection of the one end of the first resonator 4 and the other end grounded via a terminal switch 7. When the terminal switch 7 is turned off, resonance occurs at a frequency at which the sum of the line lengths of the first resonator 4 and the second resonator 6 equals to a quarter of the wavelength. When the terminal switch 7 is turned on, resonance occurs at a frequency at which a half of the sum of the line lengths equals to a quarter of the wavelength.