We have elaborated a photonic band-gap structure based on nanoporous silicon, structured both in depth and in plane with a 0.45-micrometer period. Near infrared continuous transmittance spectra, compared to calculations performed by means of a coupled-mode theory, allow quantitative measurements of the optical index both in the plane and in depth, and demonstrate that the final material index is modulated up to delta n = 0.5. Wide (delta lambda / lambda = 0.1 in TM, 0.05 in TE) and efficient stop bands are measured. They are attributed to a strong selectivity at the edge coupling.