A low-pass or band-rejection filter for microwave frequencies has a substantially planar structure and is constructed of a transmission line having inductor portions and wider capacitance portions. The inductor portions are designed as linear microstrip elements having widths being varied by making areas at the sides of the linear elements superconducting. In changing the widths of the transmission line also the inductances thereof are changed accordingly. The areas at the sides of the microstrip elements include rather narrow areas located directly at the central, normal metal conductor. These narrow areas have in the non-superconducting state some electrical conductivity which can be small but still not quite insignificant in relation to that of the metal conductor. However, due to the fact that they contact the normal metal conductor only at very narrow edges instead of contacting it at a large surface they do not significantly affect the transmission characteristics of the transmission line in the normal state of the areas which can be made superconducting.