A microstrip line element is composed of a first electrode layer (10) as a substrate which is more of a metal, a dielectric layer (20) formed by oxidizing, nitriding or oxiynitriding the first electrode layer (10), a conductor layer (30) formed on the dielectric layer (20) and a second electrode layer (40) formed on the conductor layer (30). The conductor layer (30) is composed of at least conductive nanoparticles (32) and a binder resin (31).