A tunable filter for expanding tuning range includes: a housing, which has multiple cavities defined by partitions; a resonator contained in the cavity; at least one sliding member installed over the resonator; a main cover coupled to an upper portion of the housing; and at least one tuning element coupled to a lower portion of the sliding member and made of a metallic material. Tuning is accomplished by a sliding motion of the sliding member, and the resonator includes a cylindrical first conductor and a second conductor coupled to an upper portion of the cylindrical conductor, where a cross section of the second conductor is shaped as a circle with a portion removed such that an area of overlap between the tuning element and the second conductor is varied according to a sliding of the tuning element.