  We report the discovery of a protocluster at z~6 containing at least eight
cluster member galaxies with spectroscopic confirmations in the wide-field
image of the Subaru Deep Field (SDF). The overdensity of the protocluster is
significant at the 6 sigma level, based on the surface number density of
i'-dropout galaxies. The overdense region covers ~36 sq. arcmin, and includes
30 i'-dropout galaxies. Follow-up spectroscopy revealed that 15 of these are
real z~6 galaxies (5.7 < z < 6.3). Eight of the 15 are clustering in a narrow
redshift range centered at z=6.01, corresponding to a seven-fold increase in
number density over the average in redshift space. We found no significant
difference in the observed properties, such as Ly-alpha luminosities and UV
continuum magnitudes, between the eight protocluster members and the seven
non-members. The velocity dispersion of the eight protocluster members is 647
km/s, which is about three times higher than that predicted by the standard
cold dark matter model. This discrepancy could be attributed to the
distinguishing three-dimensional distribution of the eight protocluster
members. We discuss two possible explanations for this discrepancy: either the
protocluster is already mature, with old galaxies at the center, or it is still
immature and composed of three subgroups merging to become a larger cluster. In
either case, this concentration of z=6.01 galaxies in the SDF may be one of the
first sites of formation of a galaxy cluster in the universe.
