A coaxial resonator includes a dielectric block; a first inner conductor disposed in an inner surface of a first through hole which extends from a first main surface of the dielectric block to an opposite second main surface thereof, the first inner conductor being connected to a reference potential at one side thereof; and an outer conductor disposed over side surfaces of the dielectric block, the outer conductor surrounding the first inner conductor, the outer conductor being connected to the reference potential. There is a low-dielectric-constant portion in a location between the first inner conductor and the outer conductor. The low-dielectric-constant portion surrounds a periphery of the first inner conductor and is lower in dielectric constant than the dielectric block.