1. A niobium powder for capacitors, wherein the chromium content is 50 ppm by mass or less, granulated product and sintered body thereof, and producing method of those; 2. a capacitor constructed by one part electrode formed of the niobium sintered body, another part electrode and a dielectric material interposed between two electrodes, and its producing method; and 3. an electronic circuit and electronic device using the capacitor. A capacitor having good voltage resistance properties can be manufactured by using the niobium sintered body for capacitors of the present invention, wherein the chromium content is 50 ppm by mass or less.