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. ##STR00001## wherein R.sup.111, R.sup.112, R.sup.121, R.sup.122, R.sup.131, R.sup.132, R.sup.141 and R.sup.142 each independently represent a hydrogen atom, an aliphatic group or an aromatic group; R.sup.103, R.sup.113, R.sup.123, R.sup.133 and R.sup.143 each independently represent a substituent; and n.sub.103, n.sub.113, n.sub.123, n.sub.133 and n.sub.143 each independently denote an integer from 0 to 4.