The invention relates to a membrane distillation device, comprising at least one condensation/evaporation stage, which comprises at least one condensation unit and at least one evaporation unit and to which vapor is fed and through which a liquid to be concentrated flows, wherein each condensation unit comprises a first vapor chamber that is bounded at least partially by a condensation wall and to which the fed vapor is supplied and each evaporation unit comprises a second vapor chamber that is bounded at least partially by a vapor-permeable, liquid-tight membrane wall, and in each condensation/evaporation stage at least one flow channel is provided, which is formed between such a condensation unit and such an evaporator unit adjacent to said condensation unit and which conducts the liquid to be concentrated, and thus the liquid to be concentrated is heated by means of the condensation wall, and vapor arising from the liquid to be concentrated enters the second vapor chamber through the membrane wall. At least one condensation/evaporation stage is expanded by an integrated apparatus for preheating the liquid to be concentrated, which apparatus comprises at least one additional vapor chamber, to which the vapor fed to the condensation/evaporation stage is supplied and in which the vapor is condensed, whereby the liquid to be concentrated is preheated.