The purpose of this study is to establish an evaluation method for the soiling on finishing materials of the external building walls. In this paper, some basic studies are described. According to the survey of the state of soiling on external walls, various kinds of soiling were classified into some groups, and the major factors influencing the soiling were identified. In order to estimate the degree of soiling on actual external walls, a sensory test was conducted. In the sensory test, the degree of the soiling due to flowing rain was found to be high, so this study is limited to that kind of soiling. Color difference between soiled area and unsoiled area had a correlation with obtained psychological scale. Therefore, the colorimetric value was used to indicate characteristics of the soiled materials in this study. In order to obtain a basic knowledge, the flowing rain from a top beam or a water drip was observed. And it was proved that there was a relation between the total amount of flowing rain and the degree of soiling of the external walls.