The present invention provides a method for producing an organic acid using algal biomass, The present invention pertains to the production of organic acids such as succinic acid using Euglena , by means of a method containing; either a nitrogen deficient culture step in which Euglena is cultured. aerobically under nitrogen-deficient conditions or a heterotrophic culture step in which Euglena is cultured aerobically using a culture medium containing a carbon source; and an anaerobic culture step in which the culture product obtained in the nitrogen-deficient culture step is incubated under anaerobic conditions.