To obtain a new compound having extremely large ultraviolet absorbing ability, i.e., molecular extinction coefficient in a UV-A region, and useful as an ultraviolet absorber.

The objective compound is the compound of formula I (R1 and R2 are each H, an alkyl, a cycloalkyl, an aralkyl or the like; R3 is an alkyl, an aralkyl, an aryl, a cycloalkyl or the like), e.g. 5,5-dimethyl-2-(4- hydroxyphenylaminomethylene)cyclohexane-1,3-dione. The objective compound is obtained by reacting 5,5-unsabstituted- or 5,5-disubstituted-1,3-cyclohexanedione derivative of formula II with N-formamide derivative of formula III in an organic solvent (e.g. toluene) in the presence of an acid catalyst (e.g. phosphorus pentachloride) at a reaction temperature within the range of 10-60°C.