This article discusses the privately owned condominium data in Japan. The data cover comprehensive condominium characteristics and help us understand the issues and mechanism of how condominium is evaluated. This article estimates the potential amount of condominium stock using the data obtained from private companies and compares the privately owned data to the public statistics. It is found that the condominium stock obtained from the private companies approximates the stock estimated from public statistics. I further consider the issues and future perspective with the emphasis on the validity of building unit based database.