The fermentation process of sake mashing was monitored using refracto-polarimeter. Refractive index and angle of rotation of sake mashing were simultaneously measured at an actual sake brewery. Extract content dissolved in sake mashing was directly measured with no interference of alcohol, and found to be proportional to that estimated from the conventional method with a coefficient of 0.978. Alcohol content of sake mashing was also estimated from apparent Brix measured by refractometer and extract content measured by polarimeter. It was compared with that determined by the conventional analytical method. The correlation coefficient was 0.952 through fermentation.