Disclosed are a niobium solid electrolytic capacitor capable of reducing leak current that may occur in high heat treatment in a reflow process and capable of preserving the capacity before and after heat treatment, and a method for producing it. The niobium solid electrolytic capacitor comprises an anode containing an oxide of niobium monoxide or niobium dioxide and a metal of niobium or a niobium alloy, a dielectric layer formed on the surface of the anode, and a cathode formed on the dielectric layer, wherein the dielectric layer contains fluorine.