PROBLEM TO BE SOLVED : To stably obtain a rent income with building real estate.SOLUTION : The real estate investment guarantee system has a storage device 13 which is used to manage real estate and stores pieces of real estate article information, a computer 11 which is connected to the storage device and manages real estate article information, and an input/output device 12 which is connected to the computer. When the real estate article is sold according to real estate article information and a desire for a sublease of the sold real estate article is inputted from the input/output device, a guaranteed amount which should be paid for the sold real estate article and the number of years guaranteeing the guaranteed amount are found as sublease information and displayed on the input/output device. Then when the subleasing is carried out, the computer adds a sublease ON flag showing that the subleasing is performed to the real estate article information corresponding to the sold real estate article.