In this electromagnetic wave frequency filter, an electromagnetic wave of a predetermined frequency matching a resonant frequency of a resonator 4.sub.1 is transmitted from an input waveguide 2 to an output waveguide 3 through the resonator 4.sub.1, and is outputted from a drop port P3.sub.1. This filter has an input-waveguide-side reflector 21.sub.1 and an output-waveguide-side reflector 31.sub.1, which reflect the electromagnetic wave of the predetermined frequency. The electromagnetic wave frequency filter satisfies the following relation: Q.sub.inb/(1-cos .theta..sub.1)<