An activated carbon adder adds activated carbon to raw water before flocculating sedimentation, to adsorb suspended matter contained in raw water, a liquid cyclone is adapted to separate activated carbon with suspended matter adsorbed thereon, from raw water containing activated carbon, an inflow line is connected with the liquid cyclone so as to cause raw water containing activated carbon before flocculating sedimentation to flow into the liquid cyclone and swirl therein, an activated carbon discharge line discharges activated carbon spun down out of raw water containing activated carbon in the liquid cyclone, from the liquid cyclone, and a pretreated water discharge line discharges raw water having got rid of activated carbon as pretreated water from the liquid cyclone, allowing for a reduced amount of chemicals to be used, with production of a reduced amount of sludge.