PURPOSE: To obtain dialkylbenzenes, by supplying a reaction mixture of monoalkylbenzene with an olefin to the preliminary distillation zone to separate the benzene fraction, circulating the monoalkylbenzene to the reaction zone, and recovering the dialkylbenzenes from the residual fraction.

CONSTITUTION: Toluene is supplied through the line 2 and ethylene and H2 are supplied through the line 3 to the reaction zone 1, and made to react with each other using a synthetic zeolite as the catalyst. The reaction mixture is sent to the preliminary distillation zone 5. A mixture of benzene and toluene is supplied from the benzene-toluene separation zone 6 through the line 9 to the benzene separation zone 12, benzene is obtained from the line 17, and the toluene is recycled through the line 18 and taken out of the system from the boundary between the benzene-toluene separation zone 6 and the toluene-ethyltoluene separation zone 7. The mixture recovered in the toluene recovering zone 8 is sent to the monoalkylbenzene recovering zone 16, and toluene is taken out through the line 19 and sent to the purification zone 21 to obtain ethyltoluene.