PROBLEM TO BE SOLVED : To enable management and operation of a building including property management relating to a tenant who occupies the building.SOLUTION : A building system computer 10, a construction company' s terminal 12, a caretaker' s terminal 14, a real estate company' s terminal 16 and a tenant' s terminal 18 are connected to a building management server 20. Tenants' raw data are entered from the real estate company' s terminal 16 and properties raw data are entered from the construction company' s terminal 12 and these data are stored in a building management database 24. Data of electricity consumption or the like of each tenant are entered from the building system computer 10 and stored relationally to the corresponding tenant. A maintenance work data is entered from the caretaker' s terminal 14 and stored relationally to the corresponding property. The stored data in the building management database 24 are added up at appropriate intervals and offered for open viewing at each terminal.