PURPOSE: To obtain the title coloring matter easy to handle, little in the variation of solvent content with stable quality by refluxing a nonionic cyanine coloring matter intramolecularly neutralized electric charge thereof in a solvent having a specific solubility therefor under heating, in order to improve the solubility of the coloring matter.

CONSTITUTION: A nonionic cyanine coloring matter of formula I intramolecularly neutralized electric charge thereof [Z and Z' are each O, S, Se, N, CH=CH or of formula II (R3 and R4 are each alkyl); dotted line parts of -N...Z- and -N...Z'- are each nonmetallic atom group necessary for completing (sustituted) five-membered or six-membered heterocycle; R1 is 1-12C alkyl; R2 is 1-12C alkyl substituted with anionic substituent or aralkyl; L1 - L3 are each (substituted) methyne; (k) and (m) are each 0 or 1; (l) is 0 - 3] (e.g. of formulas III or IV) is heated and refluxed in a solvent with a solubility of ≤0.5wt.% for said coloring matter (e.g. methanol), thus obtaining the objective cyanine coloring matter.