A visible-light-responsive nitrogen-doped solid-acid photocatalyst HNb3O8-N is investigated via solid-state high-resolution NMR spectroscopy. The H-1 magic-angle-spinning (MAS) spectrum observed under an ultrahigh field of 21.8 T is found drastically different from that of the precursor catalyst HNb3O8. The H-1 signals of HNb3O8-N are assigned with the help of H-1-Nb-93 and H-1-N-15 two-dimensional heteronuclear correlation (HETCOR) spectroscopy. It is clarified that H3O+ ions contained in HNb3O8 change into H2O and H+, the latter of which protonates NbO6 octrahedra. Deduced from the H-1 chemical shift for the nitrogen-adjacent protons, three microstructure models for HNb3O8-N, involving the proton-exchange model, are proposed.