NOVELTY - Method for collecting microalgae, involves injecting surplus activated sludge from second precipitation tank (80) to bioflocculant collection tank (20) using inflow pump (10), installing temperature adjusting device in the bioflocculant collection tank, adjusting temperature in the bioflocculant collection tank, separating and collecting bioflocculant from the surplus activated sludge, mixing microalgae culture medium, injecting the medium to third precipitation tank (82), and coagulating microalgae by bioflocculant.    USE - The method and device are useful for collecting microalgae using bioflocculant derived from surplus activated sludge (claimed).    ADVANTAGE - The method and device are cost-effective.    DETAILED DESCRIPTION - Method for collecting microalgae using bioflocculant derived from surplus activated sludge, involves (a) injecting surplus activated sludge from second precipitation tank (80) to a bioflocculant collection tank (20) using an inflow pump (10), (b) installing aerator in the bioflocculant collection tank, adjusting nitrogen gas and air (based on property of aerobic or anaerobic microorganism) in the surplus activated sludge to a specific flow amount, injecting the gas into the bioflocculant collection tank or injecting carbon source which is a culture medium saved in the culture medium storing tank into the bioflocculant collection tank using an injecting pump along a specific transportation line, or adjusting nitrogen flow amount of microorganism through a flow amount adjusting device along specific transportation line in a nitrogen storing tank supplying nitrogen to the bioflocculant collection tank to inject it into the bioflocculant collection tank, and inducing additional production of bioflocculant produced by microorganism in surplus activated sludge, (c) installing temperature adjusting device in the bioflocculant collection tank, adjusting temperature in the bioflocculant collection tank, and separating and collecting bioflocculant from the surplus activated sludge, and (d) mixing to-be-solid-liquid-separated microalgae culture medium which is cultured in a microalgae culture medium tank (81), injecting the medium to a third precipitation tank (82), and coagulating microalgae by bioflocculant in the third precipitation tank to precipitate the microalgae. An INDEPENDENT CLAIM is included for microalgae collecting device.    DESCRIPTION OF DRAWING(S) - The drawing shows schematic view of microalgae collecting device. (Drawing includes non-English language text)    Inflow pump (10)    Bioflocculant collection tank (20)    Second precipitation tank (80)    Microalgae culture medium tank (81)    Third precipitation tank (82)