Acidic substances contained in asphalt are affect upon emulsitiability of asphalt and its adhesion with aggregates, and therfore, detemination of acidity of asphalt is of important one. However, there has not been established a method of precisely determining the acidity of asphalt. This paper reports on an improved color-indicator titration method, by which the acid number of asphalt is determined. By treating benzene solution of asphalt with alcohol, acidic constituents were selectively extracted into alcohol solution, while asphaltic matters precipitated. A color-indicator titration method then was applied to the resultant faint-colored solution. The change of color at end point was clearly observed, and the results were obtained with good reproductivity and checked well with the results by the I.P. 177/64 method. Alcohols having not more than three cabon atoms have been found suitable for extraction.