By inserting a rectangular conductive flat member into a waveguide in contact relation of their edge portions to the waveguide walls in parallel with an electric field therein, cutoff frequencies at portions of the waveguide to which the conductive flat member is inserted become higher to form a cutoff 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. are constructed on the conductive flat member by combination of the circuit elements through mutual coupling therebetween. These microwave circuits are coupled to transmission regions of the waveguide adjoining both sides of the cut-off region by circuit elements located at the end portion of the conductive flat member. Accordingly, only by inserting into the waveguide a conductive flat member on which various circuit elements are provided, desired microwave circuits of various kinds are constructed very easily inside the waveguide. Such microwave circuits are small in size and low in transmission loss due to elimination of waveguide converters conventionally used, and are suitable for massproduction due to simplicity of structure and ease of manufacturing and assembling which results from mechanical working technique of high degree being unnecessary.