PURPOSE: To provide a water-soluble complex contg. a water-insoluble org. compd. obtd. by dissolving a water-insoluble org. compd. and a solid (co)polymer of N-vinyl-2-pyrrolidone into a mutual solvent, mixing the soln. and removing the solvent.

CONSTITUTION: The complex having the solubility in water of ≥25 times the solubility of A is obtd. by dissolving (A) the water-insoluble org. compd. (e.g. sulfathiazole, furosemide, quinine) and (B) the polymer of a mol.wt. of ≥1,000, more preferably 10,000 to 40,000 which is the solid homopolymer or copolymer (the comonomer is preferably maleic anhydride, maleic acid, acrylic acid or the like) of water-soluble N-vinyl-2-pyrrolidone in the mutual solvent (org. solvent, alkaline aq. soln., etc.), at a ratio of A:B of 1:1 to 10:1, more preferably 4:1 to 7:1, mixing the soln. for a time sufficient for forming the composite and removing the solvent.