Energetic nitrogen ion was injected into a TiO2 photocatalyst in order to investigate the optimal local concentration of doped nitrogen for visible-light response. N+-implanted TiO2 samples promoted the photocatalytic activity under visible-light irradiation. N K-edge XANES of the highest activity sample indicated that N replaces the O sites near the surface, whereas in the samples of higher N+ fluence, N−O and/or N−N species formed. Depth-resolved N K-edge ELNES revealed the two types of N, depending on the concentration, and we found the local N concentration effective for visible-light response was less than ∼1 at%. Further, the spatial distributions of the different chemical states of N by energy-filtering TEM (FETEM) supported these findings.