A forward osmosis apparatus includes a diluting means 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 separating means for separating the diluted draw solution into the cation source and anion source and into water; and a dissolving means, returning the separated cation source and the separated anion source to, and dissolving the cation source and anion source in, the draw solution that has been diluted. 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.
