PURPOSE: To provide a method for producing a highly safe aluminum tris(alkylphenoxide) compound without requiring an inert solvent and its removal process, the compound usable as a catalyst for ortho alkylation reactions as such without requiring a purification process after produced.

CONSTITUTION: A mixture containing aluminum phenoxide and phenol, e.g. a reaction solution obtained by reacting aluminum with phenol in an aluminum/ phenol molar ratio of 0.01-0.5, preferably 0.03-0.2, is reacted with an olefin such as isobutene, 1-butene or propylene in an olefin/phenol molar ratio of 0.5-4.0, preferably 1.2-2.2, to produce the reaction solution containing the aluminum tris(alkylphenoxide) compound and the alkylphenol. The obtained mixture is used as such as a catalyst for the reaction of a phenol compound with an olefin in an olefin/phenol compound molar ratio of 0.5-3.0, preferably 0.8-1.3, to produce the orthoalkylphenol compound.