PURPOSE: To change the center frequency of a band-pass filter by an electric signal, and to finely adjust the characteristics of the band-pass filter for a microwave, by placing a varactor diode at a position which is coupled electromagnetically in a shielding housing, in a dielectric resonator stored in the shielded housing.

CONSTITUTION: A dielectric resonator 2 is placed in a shielded housing 1, the resonator 2 is driven by a filter inputting circuit coupling probe 31, and a signal having a prescribed passing characteristics according to a resonance characteristics is outputted by a filter outputting circuit coupling probe 32. In this housing 1 is placed a varactor diode 7 which is supported by a post (supporting member) 8, and this post 8 is made a choke structure 9 passing through the housing 1. In this way, leakage of electromagnetic waves from the housing 1 is prevented by the choke structure 9, also DC voltage from an external terminal 10 is applied to one terminal of a diode 7, and a variation of the capacity of the diode 7, according to the voltage from the terminal is electromagnetically coupled with the resonator, by which the characteristics of the band-pass filter are finely adjusted.