A transmission line which allows electromagnetic waves in a predetermined frequency range of a millimeter waveband to propagate in a TE10 models formed by a first waveguide and a second waveguide. A resonator is formed by electric wave half mirrors fixed to the first waveguide and the second waveguide. The second waveguide has a structure in which a first transmission line forming body has a plate shape and has a square hole forming the first transmission line formed to pass therethrough from one surface toward an opposite surface, a second transmission line forming body has a plate shape and has a square hole forming the second transmission line formed to pass therethrough from one surface toward an opposite surface, and the first transmission line forming body and the second transmission line forming body are connectable and separable.