A high-frequency line connection structure connects a coaxial line and a planar line. The high-frequency line connection structure includes a conductive base that is formed into a planar shape having a length that matches a length of the planar line along a lengthwise direction of a substrate, where the planar line is disposed on a surface of the conductive base, and a protrusion structure provided in a region, on the surface of the conductive base, adjacent to the coaxial line, the protrusion structure protruding from the surface of the conductive base, where the protrusion structure is in contact with a side surface of a region along the lengthwise direction of the substrate, where a ground conductive film with a smaller width out of a pair of ground conductive films of the planar line, is formed.