PURPOSE: To provide an active circuit with which a semiconductor diode can effectively perform such operations as the switching, the detection, the conversion of frequency, etc., with an NRD guide used as a signal transmission line of the circuit and also to provide a highly effective variable frequency oscillation circuit which applies a ultrahigh frequency oscillation diode.

CONSTITUTION: Plural diodes 9 or T-type diodes 11 and 11' are vertically and symmetrically placed within a vertical section of an NRD line. Otherwise a diode mounting part 8 and a choke pattern part are formed on different substrates, and the part 8 is adhered onto the sectional face of the line together with the choke pattern part bonded or vapor-deposited on the side face of the line or inserted into the sectional face formed in the signal transmitting direction of the line respectively. In addition to a circuit of such a constitution, a stand-by resonance circuit is prepared. This resonance circuit contains a varactor diode at a position near the oscillation part where a superhigh frequency oscillation diode is used. Thus a variable frequency signal is inputted to the NRD signal transmission line via the stand-by resonance circuit.