A small flow of a water with a high carbon dioxide content (normally a carbon-dioxide-saturated water) is produced using hollow-fiber membranes, and is uniformly mixed with a large flow of untreated water with constant proportions. Since the hollow-fiber membrane module has a reserve capacity for adding carbon dioxide to the water, the resistivity of the mixed water is maintained at a constant level by maintaining constant proportions of the small and large flows, even if the total flow rate fluctuates due to fluctuations in the rate of water used in a subsequent process. Accordingly, no automated control device is particularly necessary in order to maintain the resistivity at a constant level. Therefore, a compact and simple apparatus can be produced which can operate at a low cost.