  Extra-galactic X-ray absorption and optical extinction are often found in
gamma-ray burst (GRB) afterglows and they could be tracers of both circumburst
and host galaxy environments. By performing spectral analyses for spectral
energy distribution of 9 short GRB (SGRB) afterglows with known redshift, we
investigated a ratio of the equivalent hydrogen column density to the dust
extinction, N^{rest}_{H}/A^{rest}_{V}, in the rest frame of each SGRB. We found
that the distribution of N^{rest}_{H}/A^{rest}_{V} is systematically smaller
than the one for long GRBs, and is roughly consistent with the gas-to-dust
ratio in the Milky Way. This result means that the measured gas-to-dust ratio
of SGRBs would originate from the interstellar medium in each host galaxy. This
scenario supports the prediction that SGRBs occur in non star-forming regions
in the host galaxies.
