A coaxial line is provided which includes: a first columnar conductor disposed inside a multilayer substrate such that one end thereof is coupled to a first stripline and that the other end thereof is coupled to a second stripline; and one or more second columnar conductors penetrating the multilayer substrate such that one end thereof is coupled to a ground layer and that the other end thereof is coupled to a ground layer, the first columnar conductor acting as an inner conductor, and the second columnar conductors acting as outer conductors. Each of the first and second striplines is coupled to an open stub acting as resonators and a matching conductor acting as capacitance matching elements.