The adjustable resonator according to the invention has a casing, which is composed of walls, a lid and a bottom, a resonator cavity inside the casing and an internal conductor inside the resonator cavity, which internal conductor is in electric contact with the casing. The resonator further comprises a moveable adjustment piece, which comprises a conductive adjustment member, a conductive upper plate, and a dielectric support member. The adjustment member has a stem, which is vertical, and a cap as an expansion thereof. The adjustment member can be moved downwards so that its stem and the fixed internal conductor connected to the bottom of the resonator go within each other. The movement of the adjustment piece in the coaxial resonator first decreases the resonance frequency and then slowly increases it. Therefore the resonator provides a very wide adjustment area for the resonance frequency.