The invention provides a tunable filter capable of preventing the increasing of an insertion loss, where a dielectric (11) fixed to a holding member (8) in each of the stages of reentrant cylindrical cavity resonators is movably inserted into an outer conductor, and the holding members externally protruding from the outer conductor are connected to a connecting member (9). The connecting member is caused to slide or rotate, thereby changing the distances between the dielectrics (11) and inner conductors (2) to change the frequencies of the resonators at the same time. Thereby, a high-power proof is achieved, the occurrence of mutual modulation distortions is inhibited, and the center frequency of the filter can be quickly changed in a stepless manner.