A forward osmosis apparatus includes a diluting component for bringing a feed solution and a draw solution containing a cation source and an anion source in an ionized state into contact through a semi-permeable membrane and diluting the draw solution with water separated from the feed solution by means of the semi-permeable membrane; a separator for separating the draw solution that has been diluted by the diluting component into the cation source and anion source and into water; and a dissolving apparatus, returning the cation source and the anion source that have been separated by the separator to, and dissolving the cation source and anion source in, the draw solution that has been diluted, wherein the molecular weight of the cation source in an uncharged state is 31 or greater and the Henry's law constant of each of the anion source and cation source is 1.0×104 (Pa/mol·fraction) or greater in a standard state.