In a filter circuit, a conductor layer is formed on one side of a dielectric substrate, and a resonator pattern of resonators, input and sections are formed of micro strip lines on the other side of the dielectric substrate. A transmission line coupling the resonators and is also formed of a micro strip line on the other side. An open stub branches off from the transmission line, and the electric length of this open stub is set to an integral multiple of a half-wave length of a resonance wave length corresponding to a resonance frequency of the filter.