A coaxial resonator includes a first outer conductor connected to a reference potential; a dielectric block which is provided with a through hole formed so as to pass therethrough from a first side surface to a second side surface opposed to the first side surface, and is so disposed that a first main surface abuts on the first outer conductor; an inner conductor disposed in an inside of the through hole; and a second outer conductor which is shaped like a rectangular box having its one face which is opened toward the first outer conductor, the second outer conductor having an inside dimension such that the dielectric block can be housed therein so as to be spaced from its second main surface, third side surface, and fourth side surface, and being connected to the reference potential.