A fiber raw material is fractionated into a long fiber fraction, which is rich in long fibers, and a fine fiber fraction, which is rich in short fibers and fines, after which additives are mixed into the fiber fractions. The long fiber fraction and the fine fiber fraction are combined to form a mixed stock while controlling the mixing ratio in order to keep the fines content of the mixed stock at a desired value. Additives are mixed into the mixed stock after which paper is manufactured from the mixed stock on a paper machine.
