A method for manufacturing a biofuel includes; a step of adding an alkaline agent to a culture fluid in which an algae is cultured and infusing medium quality oil thereto, thereby generating and floating an emulsion; a step of evaporating moisture from the floated emulsion in a vacuum, thereby obtaining a fat and oil component and a solids of the algae and medium quality oil and filtering, thereby separating the fat and oil component of the algae, the medium quality oil and the solids; a step of fractionally distilling the separated fat and oil component of the algae and the separated medium quality oil, thermally decomposing the fractionally-distilled components by adding an alkaline agent thereto, thereby obtaining a pyrolysis oil and a pyrolysis residue, and fractionally distilling the pyrolysis oil into thermally-decomposed gas, light crude oil, medium quality oil and heavy quality oil; and a step of removing an impurity from the fractionally-distilled medium quality oil by means of solvent extraction, thereby obtaining a biodiesel fuel oil.