An Nb solid electrolytic capacitor is disclosed which comprises: an anode body made from an Nb-based material, the anode body having a nitrogen content of about 7,500 ppm to about 47,000 ppm; a dielectric layer formed over the surface of the anode body; a solid electrolyte layer formed on the dielectric layer; and a cathode body formed on the surface of the solid electrolyte layer. The Nb solid electrolytic capacitor shows small bias dependence. A method for preparing the same is also disclosed which comprises steps of: forming an anode body from an Nb-based material, the anode body having a nitrogen content of about 7,500 ppm to about 47,000 ppm; forming a dielectric layer over the surface of the anode body; forming a solid electrolyte layer on the dielectric layer; and forming a cathode body on the electrolyte layer.