A mixed liquid of two or more components different in boiling temperature from each other and soluble in or compatible with each other is evaporated into a mixed vapor having a similar composition to that of the mixed liquid by using an apparatus having a thin film evaporator (1) having a feed inlet (1a) and a delivery outlet (1b), while forcedly circulating the non-evaporated portion of the mixed multi-component liquid by withdrawing the non-evaporated mixed liquid portion from the delivery side end portion of the evaporator and returning the mixed liquid portion into the feed side end portion of the evaporator through a circulating line (3), and the resultant mixed multi-component vapor can be used as a mixed material vapor for a gas phase catalytic reaction for producing, for example, a mono and/or a di-alkylether of an aromatic dihydroxy compound.