PURPOSE: To inexpensively obtain chitosan useful as a high polymer flocculating agent, etc., in a high producing amount by adding an organic aqueous solution to an alkali-insoluble substance obtained by treating a filamentous fungi with a heated alkaline solution and extracting from the alkali-insoluble substance at high temperature.

CONSTITUTION: A filamentous fungi (preferably Absidia coerulea-IFO-5301) is treated with a heated alkaline solution (preferably NaOH aqueous solution). Then 0.5-10w/v% aqueous solution of organic acid (preferably an aqueous solution of acetic acid) is added to the resultant alkali-insoluble substance and then the suspension is stirred and mixed while keeping the temperature at 50-100°C to extract chitosan.