the invention provides biodegradable crosslinked polymers , which have a chelating effect and a dispersing effect to disperse an oily substance and an inorganic substance hardly soluble in water and are excellent in dispersion into water . the crosslinked polymers comprise ; an ingredient of molecular weight 5 , 000 or less in 50 % by weight , a bond having at least one of a group represented by the chemical formula -- co -- o -- and a group represented by the chemical formula -- co -- nh -- as a composition unit between main chains having a water - soluble oligomer structure having the functional group represented by the general formula -- coom , and showing a viscosity of 1 , 000 cp or less at 20 ° c . by a 20 % by weight aqueous solution of the crosslinked polymer .