Provided is a method for treating water containing a low-molecular-weight organic substance which enables the low-molecular-weight organic substance to be removed with certainty at a sufficient level without subjecting the water to a biological treatment. A method for treating water containing a low-molecular-weight organic substance, the method comprising passing raw water to a high-pressure reverse-osmosis-membrane separation device, the raw water containing a low-molecular-weight organic substance having a molecular weight of 200 or less at a concentration of 0.5 mgC/L or more, wherein an amount of brine discharged from a reverse-osmosis-membrane module (5a) disposed at an end of a final stage of the high-pressure reverse-osmosis-membrane separation device is 2.1 m3/(m2·D) or more.