Seafloor Massive Sulfides have been expected to be future mineral resources. The grade of valuable metallic elements in ores of Seafloor Massive Sulfides is usually small percentage. If valuable minerals can be extracted from the ores on deep seafloor, the total mining cost can be reduced significantly. The authors proposed Seafloor Mineral Processing, where ores are to be ground into fine particles and separated into concentrates and tailings on seafloor. The Seafloor Mineral Processing system consists of processing units for unit operations such as grinding and separation. To investigate the applicability of flotation, which is a method to separate ore particles by using the difference in wettability of minerals, to the separation unit, measurements of contact angles of sulfide minerals at high pressures were carried out. The results suggested that the contact angles of the minerals would have relationships with pressure in depending on the kind of minerals and solutions. In addition, applying Laser-Induced breakdown Spectroscopy (LIBS), an optical method for elemental analysis, to measurement of metal grade of ore particles handled as slurry in the processing units was also investigated. Signals assigned to copper, zinc, and lead were successfully detected in the spectra obtained from ore particles in slurry flow.