A cell culture system having a cell culture vessel, a composition controlling fluid storage vessel, a culture fluid composition controlling means having an inlet and an outlet for a cell culture fluid, an inlet-connected fluid feeding circuit from the cell culture vessel to the inlet of the culture fluid composition controlling means, an outlet-connected fluid feeding circuit from the cell culture vessel to the outlet of the culture fluid composition controlling means, a means which perfuses the cell culture fluid from the inlet-connected fluid feeding circuit to the outlet-connected fluid feeding circuit through the culture fluid composition controlling means, and a means which controls the amount of fluid in the cell culture vessel, in which compositions of the cell culture fluid in the cell culture vessel and compositions of the composition controlling fluid in the composition controlling fluid storage vessel can be controlled in a continuous manner.