MB4-03, an $alpha$-amylase inhibitor was isolated from the culture broth of Streptomyces sp. DMCJ-49 and purified through ion-exchange chromatography, adsorption, and gel filtration. The results of various instrumental analyses showed that the inhibitor was one of oligosaccharides that had glucoses as its major component and that its molecular weight was about 2000. And one methyl group which seemed to be related with the inhibitory activity of this compound was identified. From the CMR spectrum, it was elucidated that this compound was composed of $alpha$ -D-glucopyranoses which were linked together by $alpha$ (I -, 4) bond configuration. As the inhibitory effect of this compound was reduced after incubation with $eta$-amylase, the maltose units was seemed to exist at non-reducing terminal side of it.