Five oligosaccharides were isolated from the hydrolysate of konjac (Amorphophallus konjac) glucomannan by a purified ${eta}$-mannanase from Trichoderma sp. These oligosaccharides were identified as M-M, G-M, M-G-M, M-G-M-M, and M-G-G-M; where G- and M- represent ${eta}$-1,4-D-glucopyranosidic and ${eta}$-1,4-D-mannopyranosidic linkages, respectively. The mode of action of the mannanase on the glucomannan is discussed on the basis of the structure of the above oligosaccharides.