The conventional polysaccharides having an immunostimulating activity have been used as medicinal agents health foods and additives for livestock and fish feeds. However most of the conventional polysaccharides do not exert a satisfactory immunostimulating activity. In these situations the development of a composition which is inexpensive and exerts a high immunostimulating activity has been demanded. The present invention provides: a polysaccharide which is originated from an insect and has an immunostimulating activity a composition containing the polysaccharide and an immunostimulating agent containing the polysaccharide as an active ingredient. The polysaccharide according to the present invention preferably has at least rhamnose as a constituent sugar.
