The present invention relates to a fresh water generation method using a water treatment apparatus, the method including feeding water to be treated into a membrane element including a reverse osmosis membrane or a nanofiltration membrane to separate into concentrate and permeate, in which the method includes, under operation of the apparatus, adjusting a concentrate flow rate and/or a permeate flow rate based on a water quality index of the water to be treated and a water quality index of combined water prepared by combining the concentrate and the permeate at a ratio based on a predetermined permeate recovery rate, so that the water quality index of the water to be treated falls within a tolerance on the water quality index of the combined water.