A method for producing an optical imaging contrast agent includes the following process steps: (a) reacting an albumin with a specific organic dye or a salt thereof in an aqueous solution to prepare an aqueous solution containing a combined form of the specific organic dye and the albumin (b) adjusting the pH of the aqueous solution containing the combined form to 7.2 or less (c) adding an ester to the aqueous solution having a pH of 7.2 or less and stirring the mixture, thereby preparing a liquid in which an aqueous solution phase and an organic solvent phase containing the ester are separated from each other and (d) extracting an aqueous solution containing the combined form from the phase-separated liquid.
