A frequency-tunable filter using a sliding system is disclosed. The frequency-tunable filter includes: a housing, in which a multiple number of cavities are defined by partitions; a sub-cover, which is coupled to an upper portion of the housing and in which a guide groove is formed; at least one sliding member installed in the guide groove; a main cover coupled to an upper portion of the sub-cover; a resonator held in the cavity; and at least one tuning element coupled to a lower portion of the sliding member to be inserted inside the housing, where tuning is achieved by a sliding movement of the sliding member, and at least one first guide member is coupled to at least one side surface of the sliding member such that the first guide member guides a sliding movement by way of contact with the side surface of the guide groove.