Three kinds of oligosaccharides were obtained from the hydrolysate of brown copra meal galactomannan by a purified extracellular ${eta}$-mannanase from Trichoderma sp. These oligosaccharides were identified as Man-Man, ${Gal^2}{Man_3}(6^2 mono-O-{alpha}-D-galactopyranosyl-4-O-{eta}-D-mannotriose)$, and ${Gal^2}{Man_6}(6^2-mono-O-{alpha}-D-galactopyranosyl-4-O-{eta}-D-mannohexaose)$, where Gal- and Man-represent ${alpha}$-1,6-D-galactosidic and ${eta}$-1,4-mannosidic linkages, respectively. The mode of action of ${eta}$-mannanase on brown copra meal galactomannan is described on the basis of the structure of these oligosaccharides.