A harmonic suppression resonator comprises a plurality of waveguide resonators that resonate in TE mode, in which harmonic suppression resonator, adjoining resonators are coupled via a plurality of coupling windows. Four coupling windows 33bc1, 33bc2, 33bc3 and 33bc4 are provided between a resonant region 51b and a resonant region adjoining the resonant region 51b. These coupling windows allow fundamental wave modes of the adjoining resonators to be coupled mainly by magnetically coupling. The coupling windows 33bc3 and 33bc4 allow second harmonic modes of the adjoining resonators to be electrically coupled, and the coupling windows 33bc1 and 33bc2 allow the second harmonic modes of the adjoining resonators to be magnetically coupled. By causing the amount of the electrically coupling and the amount of the magnetically coupling to be substantially equal, the coupling of the second harmonic modes is negated, whereby propagation of the second harmonic is blocked.