We present the discovery of three protoclusters at z∼3-4 with spectroscopic confirmation in the Canada-France-Hawaii Telescope (CFHT) Legacy Survey Deep Fields. In these fields, we investigate the large-scale projected sky distribution of z∼3-6 Lyman break galaxies and identify 21 protocluster candidates from regions that are overdense at more than 4σ overdensity significance. Based on cosmological simulations, it is expected that more than 76% of these candidates will evolve into a galaxy

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... er of at least a halo mass of 10^14 M_ at z=0. We perform follow-up spectroscopy for eight of the candidates using Subaru/FOCAS, KeckII/DEIMOS, and Gemini-N/GMOS. In total we target 462 dropout candidates and obtain 138 spectroscopic redshifts. We confirm three real protoclusters at z=3-4 with more than five members spectroscopically identified, and find one to be an incidental overdense region by mere chance alignment. The other four candidate regions at z∼5-6 require more spectroscopic follow-up in order to be conclusive. A z=3.67 protocluster, which has eleven spectroscopically confirmed members, shows a remarkable core-like structure composed of a central small region (<0.5 physicalMpc) and an outskirts region (∼1.0 physicalMpc). The Lyα equivalent widths of members of the protocluster are significantly smaller than those of field galaxies at the same redshift while there is no difference in the UV luminosity distributions. These results imply that some environmental effects start operating as early as at z∼4 along with the growth of the protocluster structure.