A powder composition for a capacitor comprising a tantalum or niobium and a compound having a silicon-oxygen bond, at least a part of which may be nitrided and which has an average particle size of from 0.1 to 5 m; a sintered body using the composition; and a capacitor constituted by the sintered body as one part electrode, and another part electrode. A capacitor favored with high reliability, lower dissipation level of power and smaller deterioration of capacitance than conventional tantalum capacitors using tantalum can be produced by using a sintered body of the powder composition for a capacitor.