A resonator is formed by forming a microstrip line having an electrical length corresponding to a .lambda./2 wavelength on a dielectric substrate, forming both side portions of the microstrip line from the center thereof into spiral shapes, making the orientations of the spirals' opposite each other, making outer-side portions of the spiral shapes on both sides, inclusive of the central portion of the microstrip line, linear in shape overall, and making linear in shape a portion of prescribed range from the end portion of each spiral shape.