We have investigated the self-assembling process of cellulose artificially synthesized via enzymatic polymerization as one of general problems of chemical reactions at specific sites and reaction-induced self-assembling process of reaction products in the context of nonequilibrium phenomenon and pattern formation. The chemical reaction and the self-assembling process were explored at real time and in-situ by a combined small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), ultra-SANS, and ultra-SAXS method, together with wide-angle X-ray diffraction and field-emission scanning electron microscopy. The results revealed the following pieces of new evidence: (i) Even in the aqueous reaction medium free from monomers, enzymes (cellulase) as a catalyst aggregate themselves into associations with characteristic lengths larger than 200 nm. (ii) Cellulose molecules created at each active site of enzymes associate themselves around the enzyme associations into cellulose aggregates having surface fractal dimensions D-s, increasing from 2 to 2.3 with reaction time. (iii) The fractal structure formed at the end of the reaction extends over a surprisingly wide length scale ranging from similar to 30 nm to similar to 30 mu m (3 orders of magnitude). This unique self-assembly of the reaction products is proposed to be caused by the following factors: (a) an extremely large number of polymers are repeatedly created at an active center of the catalyst in the enzyme association; (b) the polymers formed keep springing out from the narrow space in the catalyst, where the active center locates, toward the reaction medium where (c) the polymers formed associate themselves into aggregates because they are insoluble in the medium.