Abstract: A gas concentration adjusting agent which allows a low oxygen concentration environment, such as an ischemic environment for tests of cultured cell damage caused by low oxygen or the like, to be conveniently created, as well as a method and a system for adjusting gas concentrations using this easily handled agent, are provided. During the culture of cultured cells under low oxygen-ischemic culture conditions in a sealed container, the gas concentration adjusting agent consisting of water, a carbon dioxide absorbent, and an ascorbic acid-based, carbon dioxide-generating type of oxygen-absorbing composition is sealed inside the sealed container, so as to adjust the oxygen concentration in the culture environment to no more than 1% and the carbon dioxide concentration to between 3 and 7%.