NOVELTY - Offshore structure comprises floating structure body (100); carbonic acid storage tank(210) and organic matter storage tank mounted on floating structure body for respectively storing carbon dioxide and organic waste necessary for microalgal growth; microalgae culture facility (300) for storing and culturing microalgae; fuel extraction plant (400) for extracting biofuel from culture microalgae; fuel purification plant to remove impurities of biofuel extracted from fuel extraction plant; and microalgae culture tubes (700) fixed to microalgae culture facility.    USE - The structure is useful for microalgal biofuel production (claimed).    ADVANTAGE - The structure comprises maximizes the power of microalgae culture in waste organic nutrients without the need to artificially extend the size of the microalgae for culturing plant growth and a large amount of microalgae; and can reliably carry out microalgae cultivation operations without high waves depending on the weather changes and can move to a high gas field region with culture efficiency of microalgae.    DETAILED DESCRIPTION - Offshore structure comprises a floating structure body (100); a carbonic acid storage tank (210) and an organic matter storage tank mounted on the floating structure body for respectively storing carbon dioxide and organic waste necessary for microalgal growth; a microalgae culture facility (300) for storing and cultured microalgae; a fuel extraction plant (400) for extracting biofuel from the culture microalgae; a fuel purification plant to remove impurities of biofuel extracted from the fuel extraction plant; and microalgae culture tubes (700) fixed to a microalgae culture facility to promote culture efficiency by mixing carbon dioxide and organic matter in waste microalgae in a closed space.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of the offshore structure for microalgal biofuel production.    Structure body (100)    Carbonic acid storage tank (210)    Microalgae culture facility (300)    Fuel extraction plant (400)    Microalgae culture tubes (700)