aggregating agent are bonded to form flocs however the size of flocs formed here is not enough to sediment the algae and small flocs of the algae dispersed in the liquid are observed at this time then a cationic polymer aggregating agent is added thereto examples of the cationic polymer aggregating agents which can be used in the method of the present invention include a homopolymer of a cationic monomer a copolymer of a cationic monomer and a nonionic monomer condensed polyamine polyvinylamine polyvinylamidine poly meth allylamine dicyandiamide formalin condensate polyethyleneimine polyvinylimidaline polyvinyl pyridine a diallylamine salt sulfur dioxide copolymer a poly dimethyldiallylammonium salt sulfur dioxide copolymer poly dimethyldiallylammonium salt a polydimethyldiallyl ammonium salt acrylamide copolymer and an allylamine salt polymer as the cationic polymer aggregating agents which can be used in the method of the present invention particularly preference is given to the homopolymer of a cationic monomer and the copolymer of a cationic monomer and a nonionic monomer in the homopolymer of a cationic monomer or the copolymer of a cationic monomer and a nonionic monomer examples of the cationic monomer include alkylaminoalkyl methacrylate including dimethylaminoethyl methacrylate dialkylaminoalkyl acrylate including dimethylaminoethyl acrylate and diethylaminoethyl acrylate and a neutralized salt or quaternary salt thereof on the other hand examples of the nonionic monomers include acrylamide methacrylamide methacrylonitrile and vinylacetate as used herein alkyl denotes c 1 6 alkyl such as methyl ethyl n propyl isopropyl n butyl isobutyl t butyl pentyl and hexyl c 2 6 alkenyl such as vinyl 1 propenyl 2 propenyl 1 butenyl 2 butenyl 3 butenyl 1 methyl 2 propenyl 1 pentenyl 2 pentenyl 3 pentenyl 4.