PURPOSE : A system for selling long real estates for sale by auction on the Internet and a controlling method thereof are provided to enable a seller to use both sale and auction procedures and a buyer to receive a legal responsibility for surety.CONSTITUTION : A member registers information with respect to his/her real estate. If there is no transaction for a certain period of time during the real estate is purchased, an auction is automatically performed. A server(1) enables the lowest bid and rights to be inquired. An auction database(2) stores articles for auction. A plurality of user computers(41-4n) access the server(1) through the Internet and participate in the tender. When a real estate is registered and provided as an article for sale, but not purchased for a predetermined period of time, an auction is automatically processed under legal responsibility for surety, whereby a seller can use both the sale and auction procedures and a buyer can receive the responsibility for surety.? KIPO 2002