Abstract: A diamond pn junction diode includes a p-type diamond thin-film layer formed on a substrate and an n-type diamond thin-film layer formed by forming a high-quality undoped diamond thin-film layer on the p-type diamond thin-film layer and ion-implanting an impurity into the high-quality undoped diamond thin-film layer, or alternatively includes an n-type diamond thin-film layer formed on a substrate and a p-type diamond thin-film layer formed by forming a high-quality undoped diamond thin-film layer on the n-type diamond thin-film layer and ion-implanting an impurity into the high-quality undoped diamond thin-film layer.