In this study, we evaluated the level of diversity of street facades using Simpson's diversity index, a measure of diversity frequently utilized in the field of ecology. The following results are provided. (1) We extracted two types of street facades: one represented a type of diversity, where in the composition ratio of each color was almost even for the entire street facade; the other represented a type of unity, such that the dominant material accounted for more than 39% of the composition ratio within the street facade. (2) The physical and psychological values of diversity indicate a correlation. Consequently, the method of evaluating the level of diversity of the street facades using Simpson's diversity index appears to yield useful findings. The results indicate that in order to increase the attractiveness of streetscapes, it is important to recognize a dominant color or material of each street facade.