A filter element includes a flat dielectric substrate, a ground electrode on a back main surface of the dielectric substrate, resonant lines each having a shorting end in the vicinity of a border between a side of the dielectric substrate and the ground electrode and extending from the side to a front main surface of the dielectric substrate. The resonant lines and the ground electrode constitute strip-line resonators. In any of the strip-line resonators, the line width of the resonant-line side portion is different from the line width of the resonant-line main-surface portion.