A process for preparing beta-fructofuranosidase enzyme and a process for producing fructooligosaccharides, in which the preparation of the enzyme is obtained by cultivating the fungus Aspergillus niger, either wild or mutated, in a preferably semi-solid culture medium, in order to produce an extracellular enzyme, which is submitted to transfructosylation for producing fructooligosaccharides comprising sugars which are formed by one unit of sacrose and by two, three and four units of fructose.