To provide an energy-saving method for adjusting a dissolved oxygen concentration in a water tank where an oxygen generation type photosynthetic organism lives even at night in an apparatus including the water tank where an oxygen generation type photosynthetic organism such as algae lives.SOLUTION: A method for adjusting a dissolved oxygen concentration of water in a water tank in an apparatus having the water tank where an oxygen generation type photosynthetic organism lives includes: a step for storing part of high dissolved oxygen concentration water acquired by the photosynthesis by an oxygen generation type photosynthetic organism in a first storage tank； and a step for supplying the high dissolved oxygen concentration water to the water tank from the first storage tank when the dissolved oxygen concentration of the water in the water tank decreases.SELECTED DRAWING: Figure 1【課題】藻類などの酸素発生型光合成生物が生息する水槽を有する装置において、酸素発生型光合成生物が生息する水槽の溶存酸素濃度を夜間でも省エネルギーで調整可能な方法を提供する。【解決手段】酸素発生型光合成生物が生息する水槽を有する装置において水槽内の水の溶存酸素濃度を調整する方法であって、酸素発生型光合成生物が光合成することにより得られる高溶存酸素濃度水の一部を第１貯留槽に貯留する工程と、水槽内の水の溶存酸素濃度が低下した際に第１貯留槽から高溶存酸素濃度水を水槽に供給する工程とを含む、溶存酸素濃度の調整方法。【選択図】図１
