To obtain ketoisophorone with a high conversion ratio and a high selectivity in the oxidation of β-isophorone.

Ketoisophorone or its derivatives are prepared by oxidizing β-isophorone or its derivatives with molecular oxygen in the presence of the catalyst comprising a cyclic base and a complex of a transition metal with N,N'-disalicylidene diamine while controlling the moisture content in the reaction system to ≤1 wt.% at the initial stage of the reaction. The oxidation catalyst system is also constituted by further adding a compound containing a basic nitrogen to the above mentioned catalyst. The complexes of N,N-disalicylidene 2-5C alkylene diamine with manganese, iron, cobalt, copper, vanadium or the like can be used as the complex. Aliphatic or aromatic compounds having at least two nitrogen atoms can be used as the cyclic base. Schiff bases such as imino compounds and anil compounds can be used as the compound containing the basic nitrogen, and the molar ratio of the compound containing the basic nitrogen to the complex is in the range of 0.1:1-20:1 (the former : the latter).