To understand the structure formation in the Universe, over-density regions are a key component. This is because, based on the CDM cosmology, they should be corresponding to density peak of dark maters. So far, various surveys for high-z over-density regions have been performed focusing on the density excess of galaxies. However, in the high-z Universe, large fraction of baryons in galaxies could be in the gas phase. Consequently, if most of baryons in galaxies are still in the gas phase, they could be missed by previous surveys that focused on the stellar emission of galaxies. We are focusing on the damped Ly-alpha absorption (DLA) systems which is a class of quasar absorption-line systems to study about gases at high-z. We can find DLAs regardless of the luminosity of their stellar emission. Therefore we can effectively search for over-density regions without relying on the stellar emission.Based on the BOSS DLA catalog, we actually found some possible ``DLA clusters'' where there are some DLAs in very narrow regions. Those possible clusters may be corresponding to the galaxy over-density regions. To examine whether or not those possible clusters are really over-density regions, narrow-band imaging observations are useful since they trace Lyman-alpha emitters that are generally characterized by small stellar mass.In this talk, we report the results of our Subaru/Suprime-Cam narrow-band observation for Lyman-alpha emitters around a possible DLA cluster.