(Proposal ID) S11B-076 (PI) Toshikawa, Jun (Proposal Title) Searching for a Protocluster at z~6 (Abstract) We found an overdense region of i'-dropout objects in the SXDS. It includes 23 i'-dropouts identified within the 2sigma overdensity contour, corresponding to a total density contrast of more than 4sigma over the background field. We propose multi-objects spectroscopy with FOCAS targeting all objects within this overdensity. If our spectra confirm these objects are at the same redshift, this is one of the most distant known protocluster. We will be then able to discuss the natures of cluster formation and galaxy evolution from this primitive structure. The size, dynamical mass of protocluster can be derived. We already found i'-dropouts protocluster in the SDF. To compare the protocluster in SXDS with in SDF, we will learn the distinction of the earliest structure. The properties of the earliest protocluster are compared with those of low redshift protocluster, to reveal how clusters are formed. And also, the properties of member galaxies, such as star formation rates, are compared with field galaxies in low and high redshift. From this comparison, we investigate how the earliest structure in the universe influenced galaxy evolution. Through this proposed observation, we would like to approach to the cluster formation history and environmental effects on galaxy evolution.