A solid electrolytic capacitor has an anode composed of a niobium substrate and a niobium nitride layer, and a dielectric layer composed of niobium oxide is formed on the surface of the niobium nitride layer. In the solid electrolytic capacitor, the nitrogen content based on the total weight of the niobium substrate, the niobium nitride layer, and the dielectric layer is preferably not less than 0.001% by weight nor more than 0.2% by weight. In another solid electrolytic capacitor, an anode is formed by sintering thermally treated niobium powder in a nitrogen atmosphere. A dielectric layer composed of niobium oxide is formed on the surface of the anode. In the solid electrolytic capacitor, the composition ratio X of the niobium powder (NbNX) is preferably not less than 0.05 nor more than 1.0.