The present invention is to provide, in a combined system of a bioethanol producing device and an SOFC, a method that is capable of further enhancing the electric power generation efficiency of the SOFC, and is also capable of achieving further reduction of the energy required for distillation of the fermented liquid. A part of an anode off-gas is refluxed to the water-containing ethanol vapor line from the mash column to the reforming device at a reflux ratio ((flow rate of reflux gas)/(flow rate of (anode off-gas)−(reflux gas))) of from 1 to 2. The ethanol concentration of the water-containing ethanol vapor is controlled by refluxing, to a range of from 25 to 35% by weight with water contained in the anode off-gas of the solid oxide fuel cell.