Abstract: The present invention provides a method for producing high molecular-weight fucoidan, which is expected to be promising as a cosmetic composition or an agent in the field of dermatology, and a high molecular-weight fucoidan-containing composition. A component containing fucoidan having a weight average molecular weight of 1,000,000 to 2,000,000 obtained through the steps of performing hot water extraction of a mozuku alga body of the genus Cladosiphon at pH 6.0 or higher, which ranges from neutral to alkaline, removing low-molecular weight compound through ultrafiltration, and adjusting a final pH to 6.5 or higher is effective as a cosmetic composition or an agent in the field dermatology, and is also excellent in storage stability.