PURPOSE: To facilitate the manufacturing of a small BPF by shorting internal and external conductors at both edges of a constant length circular coaxial line, peeling off a part of the external conductor along a circular circumference, grounding one side, peeling off the other part and forming an input output terminal.

CONSTITUTION: At both edges of the coaxial line filling a uniform dielectric 4 between an internal conductor 1 and external conductors 2 and 3 with the length of a constant length, internal and external conductors are shorted. The external conductor is peeled off along the circular circumference at an intermediate part, separated to first and second external conductors 2 and 3 and the second external conductor 3 is grounded. At the first external conductor 2, feeding parts 19 and 20 surrounded by two peeling-off parts 14 and 15 are provided and the feeding part 19 is made into an input terminal and the feeding part 20 is made into an output terminal. Between the external conductors 2 and 3, a capacitor C1 is formed and capacitors C01 and C02 are formed between the first external conductor 2 and the feeding parts 19 and 20. The internal conductor 1 forms an inductance L1 in accordance with the length. Only by, for example, a 1/8 dielectric element much shorter than a 1/4 wavelength, a BPF is formed. Thus, the miniaturization of a filter can be easily realized.