It was found that oxide-ion conductivity of Nd-doped CeO2 (Ce1-xNdxO2-x/2) increased with an increase in Nd content x up to x = 0.2, showing a maximum value at x = 0.2, and then decreased with x. In order to clarify the origin of the decrease in the conductivity of the heavily Nd-doped CeO2, Rietveld analyses of the X-ray diffraction and neutron diffraction data were carried out, assuming two phases with oxygen deficient fluorite-type and rare-earth C-type structures (0.45 ≤ x ≤ 0.7). The oxide-ion conductivity of the heavily Nd-doped CeO2 is discussed in relation to the formation of the rare-earth C-type structure, which derives from the oxygen ordering of the defect fluorite-type structure.