In the talk I would like to present our efforts to probe a protocluster environment around 4c+23.56 by Subaru 8.2m Telescope on Mauna Kea, Hawaii. Our wide-area near-infrared narrowband observation shows that the field contains significant excess of the emission-line objects compared to the general fields. Assuming that they are the member of the protocluster at z=2.5, we show that level of the star-formation activity is as high as the field counterparts. The MIPS detection from the most of those emitters also probes the active starformation. Our detected objects are generally bright in K, implying that they are massive galaxies. Our result imply that we may gradually entering into the era of the massive cluster-dominant member formation. I would also like to show a bit about our recent result for the search of the large-scale structure around 4C+23.56.