Disclosed is a non-reducing end unsaturated mannuronic acid oligosaccharide having a molecular weight of 100-3000 Da, which is obtained by lysing polymannuronate as a substrate with alginate lyase, and provided are: a non-reducing end unsaturated mannuronic acid oligosaccharide; and pharmaceutical compositions for alleviating, preventing, or treating obesity, diabetes, and climacteric syndrome, and probiotics for promoting intestinal beneficial bacteria, the compositions and probiotics containing, as an active ingredient, the non-reducing end unsaturated mannuronic acid oligosaccharide, so that the antiobesity and antidiabetic effects, estrogen activity, and intestinal microflora controlling effect of the non-reducing end unsaturated mannuronic acid oligosaccharides are remarkably excellent as compared with non-reducing end saturated mannuronic acid oligosaccharides.