A connection structure (3) of a high-frequency transmission line according to this invention includes a columnar central conductor (7) having one end connected to a coaxial line and the other end connected to a planar transmission line, a first outer conductor (41) arranged on a side of the one end of the central conductor coaxially with the central conductor, a first dielectric body (42) filled between the first outer conductor and the central conductor, a second outer conductor (61) arranged on a side of the other end of the central conductor coaxially with the central conductor, a second dielectric body (62) filled between the second outer conductor and the central conductor, a third outer conductor (51) arranged between the first outer conductor and the second outer conductor coaxially with the central conductor, and a third dielectric body (52) filled between the third outer conductor and the central conductor. In a direction perpendicular to each of an axial direction of the central conductor and a direction perpendicular to the planar transmission line, a shortest distance between the central conductor and the first outer conductor is longer than a shortest distance between the central conductor and the third outer conductor, and the shortest distance between the central conductor and the third outer conductor is longer than a shortest distance between the central conductor and the second outer conductor.