A filter of an embodiment includes: a microstrip-line planar filter that includes an input line, resonators, and an output line, and has a passband with a center frequency f0; a metal case housing the planar filter; and structural components that include dielectric components, the structural components arranged in the metal case at an interval in the traveling direction of electromagnetic waves from the input line to the output line or in a direction perpendicular to the wavefront of the standing waves generated by the electromagnetic waves resonating in the metal case, the interval being ⅕ to ½ wavelength in terms of the electrical length of the center frequency f0.