The study investigates to isolate the oligosaccharides from the hydrolysate of konjac(Amorphophallus Konjac) glucomannan and copra galactomannan by a purified extracellular β-mannanase from Penicillium purpurogenum No. 618, and to analyze the structure of the oligosaccharides isolated, and simultaneously to comment on the specificity of the β-mannanase to the glucomannan and galactomannan, based on the structure of the oligosaccharides. 1) Isolated seven kinds of oligosaccharides obtained from the enzymatic hydrolysate of the glucomannan. The oligosaccharides were identified as M-M, G-M, G-M-M, M-G-M, G-G-M, M-G-M-M and M-G-G-M ; where G and M-represent β-1, 4-D-glucopyranocidic and β-1, 4-D-mannopyranosidic linkages, respectively. 2) The enzyme produced four kinds of galactomanno-oligosaccharides in which one or two or three α-galactosyl branching linked at O-6 of the mannosyl residue. In addition, the galactomanno-oligosaccharides did not bear α-galactosyl branching at both of the non-reducing-end and reducing-end in their structure. Form above result, it is concluded that the enzyme has the hardness toward the hydrolysis of both sides of the mannosyl residues having α-galactosyl branching.