A method of producing a near-infrared absorbing dye compound, useful for image forming materials, infrared heat-sensitive recording devices, optical film materials, and the like, containing a process of reacting a compound represented by formula (I) with peroxomonosulfuric acid or its salt. wherein R111, R112, R121, R122, R131, R132, R141 and R142 each independently represent a hydrogen atom, an aliphatic group or an aromatic group; R103, R113, R123, R133 and R143 each independently represent a substituent; and n103, n113, n123, n133 and n143 each independently denote an integer from 0 to 4.