An object of the present invention is to provide a method for performing fluid surface-floating culture of microalgae without supplying new seed algae. Microalgae on the bottom surface are used as seed algae. That is, there is provided a method for culturing microalgae which includes: a first culture process of culturing microalgae in a medium within a culture vessel, forming a biofilm on the liquid surface of the medium, and maintaining the microalgae on the bottom surface of the culture vessel; a process of collecting at least a part of the biofilm on the liquid surface formed in the first culture process and leaving at least some microalgae on the bottom surface inside the culture vessel; and a second culture process of culturing the microalgae remaining on the bottom surface within the identical culture vessel, and forming a biofilm on the liquid surface of the medium. In addition, the present invention provides novel microalgae which can form a biofilm on the liquid surface.