A connection structure includes an external conductor of a coaxial cable which is electrically connected to a ground conductor installed at a surface of a planar circuit substrate, and an internal conductor of the coaxial cable which is connected to a micro strip line installed at the other surface of the planar circuit substrate. The internal conductor of the coaxial cable is welded to a conductive pattern of the micro strip line. The conductive pattern of the micro strip line includes a conductive welding portion welded to the internal conductor of the coaxial cable, a conductive landing portion extending from the conductive welding portion in a predetermined width, and an impedance converting portion whose width increases as it extends from the conductive landing portion.