PROBLEM TO BE SOLVED: To reduce the use of a chemical agent for cleaning when a reverse osmosis treatment apparatus is cleaned to thereby improve the cleaning efficiency.SOLUTION: A reverse osmosis treatment apparatus, and a method for cleaning a reverse osmosis treatment apparatus have a first pressure vessel 80 to primarily treat a water to be treated, a second pressure vessel 82 to secondarily treat the water, a first cleaning liquid storage tank 110 to store a cleaning liquid to clean the first pressure vessel 80, and a second cleaning liquid storage tank 112 to store a cleaning liquid to clean the second pressure vessel 82, wherein in the first pressure vessel 80 and the second pressure vessel 82 each, one reverse osmosis membrane element having a reverse osmosis membrane is arranged, or a plurality of them are connected and arranged in series through water collecting piping through which permeated water flows; and the first cleaning liquid storage tank 110 is connected to a first concentrated water discharge pipe installed in the first pressure vessel 80, and the second cleaning liquid storage tank 112 is connected to a primarily treated water introducing pipe installed in the second pressure vessel 82.