Provided is a method for producing monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms, the method including a cracking reforming reaction step of bringing feedstock oil into contact with a catalyst to effect a reaction; a step of purifying and recovering monocyclic aromatic hydrocarbons separated from the reaction step; and (1) a step of hydrogenating a heavy fraction separated from the reaction step; a dilution step of returning a portion of the hydrogenation product as a diluent oil to the hydrogenation step; and a step of returning the hydrogenation product to the reaction step; or (2) a step of adding a diluent to the heavy fraction separated from the reaction step; a step of hydrogenating the mixture; and a step of returning the hydrogenation product to the reaction step.