the object of the present invention is to provide a method for aggregating and separating algae dispersed in a liquid efficiently . according to the present invention , a method for aggregating and separating algae dispersed in a liquid , which comprises adding an alkaline inorganic aggregating agent to the liquid at ph of 2 . 0 to 4 . 0 in which algae are dispersed , followed by adding a cationic polymer aggregating agent , is provided . further , according to the present invention , a method for producing biofuel utilizing algae is provided .