A niobium powder for capacitors, having an average particle size of from 10 to 500 .mu.m, which is a granulated powder having an oxygen content of 3 to 9% by mass; a sintered body thereof; and a capacitor fabricated from the sintered body as one part electrode, a dielectric material formed on the surface of the sintered body, and another part electrode provided on the dielectric material. A capacitor manufactured from the sintered body of a niobium powder of the present invention is prevented from deterioration in the performance for a long period of time and has high reliability.