A method for producing an aromatic hydrocarbon easily and safely without the use of carrier-supported catalyst and is characterized in that an unsaturated compound having a double bond or bonds that are conjugated with the benzene ring is allowed to coexist in the process to alkylate a substituted aromatic hydrocarbon with olefins in the presence of 5 mM or more of metallic sodium and potassium compound to provide 3 mM or more of potassium ions relative to 1 mole of the substituted aromatic hydrocarbon, said substituted aromatic hydrocarbon having at least one alkyl group or alkylene group which has at least one hydrogen atom in the .alpha.-position relative to the aromatic ring of said substituted aromatic hydrocarbon.