There is a strong need to prevent the heat island effect. One of the important countermeasures to the heat island effect is to reduce thermal storage in buildings. The highly reflective material on outer walls and roofs is a significant way to prevent heat from penetrating indoors; such materials have been installed on the surface of buildings in Osaka, Japan. To evaluate the solar reflectance of such highly reflective material, we measured the solar reflectance of reflective roofing sheets installed on the building roofs in Osaka, Japan, over a span of time from February 2010 until September 2011, and examined the change in solar reflectance during this time.