After an aqueous solution containing, at a concentration of less than 40% by mass, an organic compound having two or more hydrophilic groups in a molecule is adjusted to contain the organic compound at a concentration of equal to or greater than 40% by mass, the aqueous solution whose organic compound concentration is adjusted to equal to or greater than 40% by mass is irradiated with an ultrasonic wave to atomize water, and is dehydrated and concentrated.