Abstract: By inserting a rectangular conductive flat member having uninterrupted straight edges into a waveguide in contact relation of their edge portions to the waveguide walls in parallel with an electric field therein, cut-off frequencies at portions of the waveguide to which the conductive flat member is inserted become higher to form a cut-off region. The conductive flat member is composed of a conductor plate or by evaporating a conductive thick film on a dielectric or magnetic plate. The conductive flat member occasionally lined with a non-conductive flat member is provided with circuit elements such as slots formed by punching and strip lines or semiconductor diodes formed by evaporation, and microwave circuits having various functions such as filter circuits, directional couplers, oscillator circuits, frequency converter circuits, frequency multiplier circuits, switch circuits, phase shifter circuits, etc.