PURPOSE: To provide a device and a method for preparing solution which do not cause large-sized equipment and where the makeup of reagents, etc., is not required over a long period of time.

CONSTITUTION: In a preparing device 1, a pure water tank 10 and a reagent tank 20 are connected to a pump 30 for sucking and delivering pure water and the original liquid of a reagent from the tanks respectively and the pump 30 is connected to a mixing column 60 for mixing two liquids through a pressure holding valve 40 on the pure water side and a pressure holding valve 50 on the reagent side and the mixing column 60 is connected to a solution tank 70 for storing solution. Level sensors are provided in the upper and lower parts of the solution tank 7. When the level reaching high level is detected by a high level sensor 71, the pump 30 is stopped by an operation stop means. On the other hand, when the level lowering to low level is detected by a low level sensor 72, the pump 30 is operated by the operation stop means. And the pure water tank 10 is connected to a pure water producing device 13 and a dissolved gas removing device 16.