A microwave filter is disclosed in which open-ended lines are connected at three or four points of a main line having input and output terminals. The open-ended lines have a length about one fourth the wavelength of an image signal frequency thereby to produce a band-stop filter characteristic with an image frequency, all or parts of the open-ended lines constitute open-ended parallel-coupled lines arranged in parallel to the main line, and the intevals of connection of the open-ended lines with the main line are properly selected. The insertion loss of the filter within the pass band of a radio frequency signal is reduced and the filter attenuation within a stop band of an image signal is increased, thus providing a compact band-stop filter which has a steep rise characteristic.