A sugar derivative derived from Aphanothece sacrum as a freshwater blue-green alga, having a mean molecular weight of 2,000,000 or more and a repeat structure of a sugar chain unit where a sugar constituent of a hexose structure and a sugar constituent of a pentose structure are conjugated together in a linear chain or a branched chain, containing a lactated, sulfated sugar as a constituent, where 2.7 or more hydroxyl groups per 100 hydroxyl groups are sulfated or sulfur element occupies 1.5% by weight or more of all of the elements, in the sugar chain unit. A sugar derivative with a novel polysaccharide structure as derived from Aphanothece sacrum is provided in such manner.