A reliable process has been developed for the fabrication of all Nb single-electron circuits, based on spin-on glass planarization. The process steps are the in situ growth of Nb/AlOx/Nb sandwich, definition of the patterns of junctions, base electrodes, and wiring by use of reactive ion etching and the planarization of a spin-on glass insulation between base electrode and wiring. A single-electron transistor made of 0.3 X 0.3 mu m(2) area junctions clearly shows the e-periodic Coulomb blockade modulation by a voltage applied to agate.