Sunlight was found to strongly induce the formation of N^ε-(carboxymethyl)lysine (CML) from glycated polylysine in the presence of Fe(III) ion. The initial step of this Fe(III)-catalyzed CML formation was noted to be similar to that of blueprint photography as was confirmed by the production of Turnbull's blue in sunlight-exposed glycated human serum albumin ferricyanide solution in the presence of Fe(III). Based on this, photoinduced oxidative C–C bond cleavage of the Amadori compound was assumed to be initiated by photochemical single electron transfer from ligand to Fe(III) in the Fe(III)–Amadori compound complex affording the Fe(II)–Amadori compound radical intermediate, which eventually yields either CML or active oxygen species. CML is thus a useful oxidative stress marker. The mechanism proposed here would explain the high accumulation of CML in lens protein and skin actinic elastosis.