NOVELTY - An ethanol/water mixture is prepared by feeding fermentable raw materials (e.g. sugar or raw materials capable of being hydrolyzed into fermentable sugars) and auxiliary substances into a reactor, separating ethanol/water mixture from reactor fermentation solution in first falling-film evaporator, and removing non-fermented matter from the reactor. Water is removed as underflow, non-volatile matter is removed from first evaporator, and water is recycled to the reactor.    USE - For preparation of ethanol/water mixture useful as constituent of fuel.    ADVANTAGE - The inventive method includes separating water in second falling-film evaporator and recycling it more than once in the second evaporator, thus minimizing the ethanol concentration of water prior to discharging the water from the evaporator. It allows the use of inexpensive and readily available raw materials e.g. wastes of food industry e.g. biowaste of stores and waste mass of bakeries, milk processing production, and potato processing. Its carbon dioxide emissions are lower compared to the prior art methods. It is more cost-efficient with respect to the investment and operating costs compared to the prior art methods. It is simpler and requires less control in use, specifically in the category less than 10000 tons of ethanol per year. It utilizes an apparatus that is simple. The invention contributes to the improvement of transportation logistics because the ethanol/water mixture can be transported to oil and gasoline terminals in conjunction with the return of gasoline trucks.    DETAILED DESCRIPTION - Preparation of ethanol/water mixture comprises feeding fermentable raw materials (e.g. sugar or raw materials capable of being hydrolyzed into fermentable sugars) and auxiliary substances into a reactor, separating ethanol/water mixture from reactor fermentation solution in first falling-film evaporator (13), and removing non-fermented matter from the reactor. The reactor fermentation solution is continuously fed into first falling film evaporator during fermentation. The vapor obtained from the evaporator is condensed, and concentrated in a second falling-film evaporator (18). Water is removed as underflow, non-volatile matter is removed from first evaporator, and water is recycled to the reactor.    INDEPENDENT CLAIMS are included for:    (1) an apparatus for preparing an ethanol/water mixture, comprising reactor, evaporator, fermentation solution discharging conduit, exhaust pipe, two falling-film evaporator, non-volatile matter discharging conduit, and condenser (16, 21, 23); and    (2) use of ethanol/water mixture as constituent of fuel, where the ethanol/water mixture is concentrated to the desired ethanol content and the concentrated ethanol mixture is mixed with the other fuel constituents to form a predetermined fuel mixture.    DESCRIPTION OF DRAWING(S) - The drawing shows the apparatus used in the invention.    Fermentation vessel (4)    Solid matter separator (8)    First falling-film evaporator (13)    Condenser (16, 21, 23)    Second falling-film evaporator (18)    Storage tank (25)