To investigate optimal condition of acid-catalyzed hydrothermal hydrolysis of Laminaria japonica, the main constituentsof Laminaria japonica such as cellulose, alginic acid and mannitol were hydrolyzed using acid-catalyzed hydrothermalreaction. Then, we proposed the re-designed experimental method including the predicted optimal conditions of the mainconstituents and performed acid-catalyzed hydrothermal hydrolysis of Laminaria japonica. The coefficients ofdetermination (R2) of Y5 (yield of reducing sugar from Laminaria japonica) were 0.877. P values of Y5 were 0.002,indicating significance, within 1% (p<0.01). The optimum reaction condition for acid-catalyzed hydrothermal hydrolysisof Laminaria japonica determined by the response surface methodology is 143.65oC of reaction temperature, 22min ofreaction time, hydrochloric acid concentration 0.108N, resulting in a production rate of 115.62mg/g-Laminaria japonica.