The present invention relates to a food composition comprising Suaeda japonica as an active ingredient for preventing or alleviating diabetes, and to a method for preparing the composition. When Suaeda japonica powder of the present invention is administered to OLETF mice in which obesity and diabetes are naturally induced during growth due to a defect of the CCK-1 gene, an increase in blood glucose and production of glycosylated hemoglobin are inhibited, and the effect of improving glucose tolerance is exhibited. Also, a high blood level of adiponectin, which relates to an improvement in insulin sensitivity and to the recovery of glucose metabolism function, is maintained, and antidiabetic activity, specifically, the effect of alleviating or preventing diabetes, is excellent.