NOVELTY - System has algae culture unit (1), solid-liquid separation unit (2) to separate the algae cultured by the culture unit from cultured water, first return unit (3) to return the cultured water separated from algae by the solid-liquid separation unit to the culture unit, useful component extraction unit (4) to separate and extract a useful component from the algae separated by the solid-liquid separation unit, residue process unit (5), second return unit (6), and a nutrient component replenishment unit (7).    USE - The system is used for production of useful substance from algae, including biogas, biofuel (claimed), hydrocarbon and medicinal substance. The algae includes Botryococcus braunii, Chlorella sp., Cryptothecodinium cohnii, Cylindrotheca sp., Dunaliella primolecta, Isochrysis sp., Monallanthus salina, Nannochloris sp., Nannochloropsis sp., Neochloris oleoabundans, Nitzschia sp., and Schizochytrium sp..    ADVANTAGE - The system produces useful substance from algae at low cost, saves energy, and has a contribution in reduction of emitting amount of carbon dioxide to atmosphere.    DETAILED DESCRIPTION - Useful substance production system has algae culture unit (1), solid-liquid separation unit (2) to separate the algae cultured by the culture unit from cultured water, first return unit (3) to return the cultured water separated from algae by the solid-liquid separation unit to the culture unit, useful component extraction unit (4) to separate and extract a useful component from the algae separated by the solid-liquid separation unit, residue process unit (5) to mineralize or solubilize the residue after separating and extracting a useful component, second return unit (6) to return the component mineralized or solubilized by the residue process unit to the culture unit, and a nutrient component replenishment unit (7) to replenish the decreased portion of a nutrient component required for culture of the algae in a culture unit.    DESCRIPTION OF DRAWING(S) - The drawing shows schematic view of system for production of useful substance from algae. (Drawing includes non-English language text)    Culture unit (1)    Solid-liquid separation unit (2)    First return unit (3)    Useful component extraction unit (4)    Residue process unit (5)    Second return unit (6)    Nutrient component replenishment unit (7)    Precoat type rotating drum type solid-liquid separator (20)    Methane fermentation apparatus (51)