NOVELTY - Method for cultivating microalgae using culture apparatus, involves adding culture solution to microalgae suspension liquid in horizontal flow path, harvesting substantially whole quantity of microalgae suspension liquid except that accommodated in the storing tank, maintaining anaerobic conditions in storing tank containing the suspension liquid and supplying gas in daytime into the first and second vertical flow path.    USE - The method is useful for cultivating microalgae in culture apparatus (claimed) and purifying organic wastewater e.g. livestock farming wastewater, and fixing carbon dioxide.    ADVANTAGE - The culture apparatus is inexpensive.    DETAILED DESCRIPTION - Method for cultivating microalgae using culture apparatus, involves adding culture solution to the microalgae suspension liquid in horizontal flow path, harvesting substantially whole quantity of microalgae suspension liquid except that accommodated in the storing tank, maintaining anaerobic conditions in storing tank containing microalgae suspension liquid and supplying gas in daytime into the first vertical flow path and second vertical flow path. The capacity of the tank is 20% or less. The horizontal flow path comprises first flow path edge portion and second flow path edge portion, where the accommodated liquid is irradiated with sunlight. The substantially perpendicular first and second vertical flow paths are connected to the first and second flow path edge portion. The tank comprises lower portion, which is connected to first vertical flow path and second vertical flow path with its upper portion sealed. The tank is equipped with exhaust mechanism. The gas is supplied to the tank using pressure injection machine. The culture apparatus of the microalgae has maximum height of the connection flow path in the boundary portion between the first vertical flow path and the storing tank, which is higher than the maximum height of the connection flow path in the boundary portion between the second vertical flow path and storing tank. An INDEPENDENT CLAIM is included for culture apparatus of microalgae, which comprises the above-mentioned unit.