To obtain fine reduced powder having a uniform grain size distribution by feeding the halide or alkoxide of tantalum or niobium to a plasma arc together with gaseous hydrogen from the lower stage nozzle in a multistage plasma torch of a DC plasma CVD device.

A cathode 7 respectively connected to a pilot power source 4 and main power sources 5 and 6, and anodes 8-1, 8-2 and 8-3 laminated in the longitudinal direction are provided, and nozzles 1 to 3 of 1 to 3 stages are provided to compose a three stage plasma torch 15. The 1st nozzle 1 is fed with an electrode gas 10 such as gaseous argon to generate a long argon column 9 of high speed in a reaction space 14. From the 2nd and 3rd nozzles 2 and 3, the halide or alkoxide of tantalum or niobium and gaseous hydrogen are fed into the arc column 9. In this way, the tantalum or niobium raw material is instantaneously reduced to produce tantalum or niobium powder in which the average grain size of primary grains is 50 to 150 nm, and, as to the grain size distribution, the standard deviation is ≤1.4 by a logarithmic normal probability distribution.