To provide a method of producing a fucoidan extract by performing processes of cleaning, boiling with a citrus fruit extract, pressure sterilization, immersion in liquor, and filtration for extracting more fucoidan highly included in seaweed of the family Spermatochnaceae, kelp, Undaria pinnatifida, and other marine algae.

For increasing extraction efficiency of fucoidan highly included in seaweed of the family Spermatochnaceae, kelp, Undaria pinnatifida, and other marine algae, cleaning with fresh water and boiling with a citrus fruit extract are carried out to extract fucoidan, and when a filtration residue is immersed in liquor, a fucoidan extract further increased in its amount can be easily obtained at low cost.