To find a new use of acetyl fucoidan and a compound derived therefrom.

The antineoplastic agent comprises acetyl fucoidan or sulfated acetyl fucoidan obtained by sulfating the acetyl fucoidan as an active component. The sulfating reaction is carried out by dispersing acetyl fucoidan in DMF and reacting with sulfur trioxide-trimethylamine. The acetyl fucoidan used as a raw material can be produced by treating Cladosiphon okamuranus TOKIDA with hydrochloric acid, neutralizing with sodium hydroxide, extracting with ethanol, subjecting to cation exchange with Amberlite(R) IR-120, and carrying out neutralization, dialysis and freeze-drying to obtain a Na-type purified acetyl fucoidan.