A method for producing D-galactose from a dissolved solution of dried whey permeate or a liquid whey permeate is disclosed. The method comprises: removing non-sugar insoluble materials from the dissolved solution of dried whey permeate or the liquid whey permeate; removing protein from the dissolved solution of dried whey permeate or the liquid whey permeate from which non-sugar solid precipitates are removed; and removing ash, salts or both from the dissolved solution of dried whey permeate or the liquid whey permeate from which the proteins are removed. Another method for producing D-tagatose is disclosed. The other method comprises isomerizing D-galactose produced by the foregoing method.