A niobium or tantalum powder of the present invention comprises aggregates in which primary particles of niobium or tantalum are aggregated, and have a pore distribution having a peak in the range from 1 to 20 .mu.m as measured by mercury porosimetry. That is, the niobium or tantalum powder of the present invention comprises aggregates having large pores, which connect with vacancies between the primary particles and facilitate the permeation of an electrolyte over the entirety of the inside of each aggregate. Accordingly, a solid electrolytic capacitor comprising an anode electrode made of the niobium or tantalum powder has high capacity and also a low ESR.