Abstract: Disclosed is a medium-molecular weight fucoidan which can be used in treating and/or preventing proliferative diseases, in particular, cancer. Specifically disclosed are a medium-molecular weight fucoidan, a method for producing the same, a medicinal composition containing the same, and use of the medium-molecular weight fucoidan in producing a drug for treating and/or preventing preventing proliferative diseases, in particular, cancer. The aforesaid medium-molecular weight fucoidan has an average molecular weight of about 6,000 to about 2,000,000, preferably an average molecular weight of about 40,000 to about 330,000, more preferably an average molecular weight of about 40,000 to about 244,000, and still more preferably an average molecular weight of about 80,000 to about 200,000.