  Charge ordering of V4+ and V5+ in NaV2O5 has been studied by an X-ray
diffraction technique using anomalous scattering near a vanadium K-absorption
edge to critically enhance a contrast between the two ions. A dramatic energy
dependence of the superlattice intensities is observed below Tc=35 K.
Consequently, the charge ordering pattern is the zigzag-type ladders with the
unit cell 2a*2b*4c, but not the chain-type originally proposed for the
spin-Peierls state. Charge disproportionation suggested in our model as the
average valence V^{4.5+-delta_c/2} is observed below T_C, showing continuous
variation of delta_c as a function of temperature.
