PURPOSE: To provide uniform dual tuning characteristic at the entire frequency band, to form a trap at a desired frequency and change interlockingly the pass band and the trap frequency, by providing a resonance line element and an open stub to input and output terminals and adjusting the degree of coupling with a microstrip line element.

CONSTITUTION: The resonance line element 36 and varactor diodes 37, 38 form the 1st resonance circuit and the resonance line element 30 and varactor diodes 41, 42 form the 2nd resonance circuit. The open stubs 43, 44 are arranged so as to be couled electromagnetically. The trap frequency A is set independently of the adjustment of the pass band B by adjusting the degree of coupling of the open stubs 43, 44 with said constitution. Further, a wide variable frequency is obtained with diodes 37∼42. Moreover, the frequency A is adjusted independently of the waveform of the band B by adjusting the degree of coupling of the stubs 43, 44. The degree of coupling of the resonance line elements 36, 40 is adjusted by changing the interval of the open stubs 45, 46 comprising the microstrip line element between the elements 36 and 40 and the waveform of the pass band is shaped.