An apparatus for converting a transmission structure includes a conductor-backed coplanar waveguide (CBCPW) transmission line comprising metal layers formed on both sides of a board, a first signal line and a first ground line formed by removing the metal layer on one side of the board according to a pattern line, a plurality of via holes formed on the first ground line, proximate to the pattern line, and a parallel transmission line which forms a second signal line and a second ground line parallel with the second signal line, by being connected at one end with the first signal line and the first ground line in a substantially perpendicular direction with respect to the board, respectively. By the parallel transmission line which is connected with the planar transmission line in a perpendicular direction, a compact-sized wideband converter can be provided at an economical price.