We investigated alginate oligosaccharide (AOS) as a simple growth promoter of Spirulina. The maximum growth promotion effect of 1,000 ppm AOS was 3.68-fold higher than that of control at 8 days of batch culture. Biomass productivity and contents of metabolites, such as phycocyanin and protein, increased with repeated batch feeding of AOS. Highest biomass productivity of 168 mg L-1d-1 was obtained at the third stage of repeated batch culture.