To suppress occurrence of contamination, efficiently culture algae in culture of algae utilizing a water source in the natural world.SOLUTION: An algae culture system 100 comprises: a culture tank 10 which houses culture solution Q including seawater or brackish algae M; a pH measuring device 20 which measures pH of the culture solution Q housed in the culture tank 10; a pH adjusting material supply device 30 as a pH adjusting device which adjusts pH of the culture solution Q and a light source device 40; and a controller 50 which controls the pH adjusting device such that pH of the culture solution Q is maintained at alkalinity at least for a predetermined time on the basis of pH of the culture solution Q measured by the pH measuring device 20.SELECTED DRAWING: Figure 2

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