Six kinds of oligosaccharides were obtained from the hydrolysate of copra galactomannan by a purified extracellular $beta$-mannanase from Penicillium purpurogenum. These oligosaccharides were identified as M-M, M-M-M, M-M, M-M-M-M, M-M-M-M-M and M-M-M-M-M-M; where G- and M- represent $alpha$-l,6-D-galactosidic and $beta$-l,4-mannosidic linkages, respectively. The mode of action of mannanase on galactomannan is discussed on the basis of the structure of these oligosaccharides.