PURPOSE: To obtain the characteristics equal to those of a multi-stage filter for a composite filter, by using a dielectric material having a prescribed dielectric constant to secure connection between the 1st and the 2nd filters using plural resonators having the shortened electrical length via a coaxial line.

CONSTITUTION: Two-stage filters F1 and F2 are connected longitudinally via a coaxial line CL having a length set so as to obtain desired phase and quantity of a reflected light. As a result, the characteristics can be obtained as shown by a solid line in the figure. The BPF filter characteristics shown by a dotted line are obtained by one of the filters F1 and F2. The filters having such characteristics are connected in two stages with compensation of the disturbance of characteristics. Thus it is possible to obtain the characteristics (shown by a solid line) equal to a filter which is previously designed as a 4-stage filter. Then a dielectric material of ≥15 dielectric constant is used to obtain a filter using plural resonators having shortened electric lengths. Thus it is possible to connect filters having individual characteristics with no intervention of a buffer means.