The method of coagulation and separation algae

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.