A filter is provided with a multilayer board incorporating a resonator formed by two ground plates opposed to each other and conductive side walls connected to the ground plates; two signal vias provided through the resonator; and two terminals connected to the signal vias to receive a pair of signals. The resonator has a first face of symmetry vertical to the ground plates. The signal vias are disposed symmetrically with respect to the first face of symmetry on a distance from the first face of symmetry.