A coplanar stripline is provided, including a dielectric substrate, and first and second strip conductors, which are each formed on the dielectric substrate, parallel to each other and separated by a predetermined distance. The corrugated structure is formed at least on one side of the strip conductors, including a plurality of slits. The slits are formed periodically on the side of each of the strip conductors, which is located in the outside portion thereof and not facing the other confronting strip conductor, so that the length direction of the slit is substantially perpendicular to the direction of the electromagnetic wave transmitted through the coplanar stripline.