Extraction of brown seaweed (Undaria pinnatifida) oil was carried out by using supercritical carbon dioxide ($SCO_2$) and ethanol as co-solvent. The flow rate of ethanol was 3.0% (v/v) as compared to that of $SCO_2$. Experiments were performed in a semi-batch flow apparatus on dried samples at temperatures from 303 to 333 K and pressures from 80 to 300 bar. Fucoxanthin and polyphenol were quantitatively analyzed by using HPLC and UV-spectrometer. The highest yields of fucoxanthin and polyphenol were shown at 200 bar, 323 K and 250 bar, 333 K, respectively. The solubility of fucoxanthin in $SCO_2$ agreed well with the Chrastil model.