  We report results of deep optical spectroscopy with Subaru/FOCAS for Lyman
Break Galaxy (LBG) candidates at z~5. So far, we made spectroscopic
observations for 24 LBG candidates among ~200 bright (z'<25.0) LBG sample and
confirmed 9 objects to be LBGs at z~5. Intriguingly, these bright LBGs show no
or a weak Ly-alpha emission and relatively strong low ionization interstellar
metal absorption lines. We also identified 2 faint (z'>25.0) objects to be at
z~5 with their strong (EW_rest>20AA) Ly-alpha emission. Combining our results
with other spectroscopic observations of galaxies at the similar redshift
range, we found a clear luminosity dependence of EW_rest of Ly-alpha emission,
i.e., the lack of strong Ly-alpha emission in bright LBGs. If the absence of
Ly-alpha emission is due to dust absorption, these results suggest that bright
LBGs at z~5 are in dusty and more chemically evolved environment than faint
ones. This interpretation implies that bright LBGs started star formation
earlier than faint ones, suggesting biased star formation.
