Provided is a semi-coaxial resonator that reduces the insertion loss in the pass band and that is made to be compact, lightweight, and low in cost. A resonator is formed by fastening a cylindrical first element with a square and planar second element at an open end (one end) of the first element. A plurality of these resonators are disposed such that the square edges are brought close to each other and the other ends of the first elements are secured to the bottom of a box-shaped case. The open side of the case that accommodates the plurality of resonators is closed by a lid.