Since surfaces of dielectric substrates for printed boards are usually roughened to make tight copper-coating, it was found out that an effective conductivity of a surface of the copper foil, attaching on the dielectric substrate, more degraded than that of an opposite surface of the copper foil, facing an air region, beyond centimeter frequencies. The transmission loss therefore degrades due to the roughness of the copper foil surface. On the other sides, it was obvious that the current density on the under surface of the copper foil decreases by symmetrically-loading metal patterns on both sides of the dielectric substrate and by biasing equi-voltage to both metal patterns, and thus the transmission loss was relatively unaffected by the roughness of the dielectric surfaces. Based on this consideration, a bi-laterally metal-loaded tri-plate line was evaluated by using a cheap FR-4 substrate, being poor material for use as millimeter wave lengths but being cost-effective.