A plane filter includes a dielectric substrate, a filter part provided on the dielectric substrate, and an input/output line connected to the filter part on the dielectric substrate. The filter part and the input/output line are provided on a front surface of the dielectric substrate. The dielectric substrate includes first and second regions. The filter part is provided in the first region, and the input/output line is provided in the second region. The first region having a first thickness in a first direction toward the front surface from a back surface at a side opposite to the front surface. The second region having a second thickness in the first direction, the second thickness being less than the first thickness. The back surface includes a step corresponding to a difference between the first thickness and the second thickness.