The present invention relates to a niobium alloy for capacitors comprising as an alloy component from 0.01 to 10 atom % of at least one element selected from the group consisting of the elements belonging to Groups 2 to 16 of the periodic table and further comprising diniobium mononitride crystals of from 0.1 to 70 mass %, wherein a powder of the niobium alloy has an average particle size of 0.05 to 5 .mu.m and a BET specific surface area of 0.5 to 40 m.sup.2/g, a granulated product of the niobium alloy having an average particle size of 10 to 500 .mu.m, a sintered body of the powder of the niobium alloy or granulated product thereof, a capacitor and a producing method thereof using the sintered body. A niobium capacitor using the powder of the niobium alloy of the present invention or a granulated product thereof has high capacitance and small leakage current value and is excellent in high-temperature property and heat resistance property.