In this paper the author shows the details of several problems related to the average sky factor of street in an urban space. These problems are as follows. (1) The regression equation for the average sky factor. (2) The relation between the average sky factor and the site area, or the road ratio. (3) The existence of the maximum average sky factor. (4) The comparative study of the street of which the building coverage ratio is 100 % and the street in which the buildings on either side will be assumed to be built in blocks of uniform height, extending continuously in both directions.