We studied a extraction and degradation of alginate from seaweed-stems using microorganism DS-02. DS-02 has a maximum growth rate at $30^{circ}C$ and the enzyme has a maximum activity of alginate extraction at $35^{circ}C.$ The yield of alginate extraction using DS-02 is about $16.0{\%}$ for 3.0 hour and molecular weight of the alginate decreased to about 1/8 of initial value after 24 hour extraction. Alginate extraction method by DS-02, compared with general alkali-extraction method, has an advantage of decreasing the molecular weight of alginate during extraction.