To extract and produce purely white high-quality fucoidan from cultured itomozuku, a kind of seaweed of the genus Nemacystus decipiens, by a simple and easy process.

Currently, fucoidan is produced from a Kombu and the like; however, since the content of fucoidan in a Kombu is low and the content of alginic acid, a polysaccharide, is high, the production process of fucoidan from a Kombu is complicated, resulting in high-price fucoidan. Moreover, fucoidan now available commercially is brown and poor in quality. Contrary to expectations, high-purity (purely white) fucoidan having a mol.wt. of 500,000 can be produced by dispersing itomozuku in an acidic soln. and stirring at room temp., though itomozuku contains alginic acid in a high content (1.5 times that of fucoidan). Because of the brown color, very low purity, and high price, a commercially available fucoidan is limited in its application; therefore, the production of high-quality fucoidan raises the value added. For producing purely white high-quality fucoidan, the use of a freeze drier is the most important.