(1) A niobium monoxide powder for a capacitor represented by formula: NbO.sub.x (x=0.8 to 1.2) and optionally containing other elements in an amount of 50 to 200,000 ppm, having a tapping density of 0.5 to 2.5 g/ml, an average particle size of 10 to 1000 .mu.m, angle of repose from 10.degree. to 60.degree., the BET specific surface area from 0.5 to 40 m.sup.2/g and a plurality of pore diameter peak tops in the pore distribution, and a producing method thereof; (2) a niobium monoxide sintered body, which is obtained by sintering the above niobium monoxide powder and, having a plurality of pore diameter peak tops in a range of 0.01 .mu.m to 500 .mu.m, preferably, the peak tops of two peaks among the plurality of pore diameter peak tops having a highest relative intensity are present in the range of 0.2 to 0.7 .mu.m and in the range of 0.7 to 3 .mu.m, respectively, and a producing method thereof; (3) a capacitor using the above sintered body and a producing method thereof; and (4) an electronic circuit and electronic device using the above capacitor.