1. A unit for saving energy for a building, comprising: an input interface that receives information from sensors assigned to the unit; a communication network capable of mutually communicating with other units; a unit-specific information storage that stores information in a format suitable for an exchange of the stored information between different units, wherein the stored information comprises a location, a structure, and an environmental condition of a target building in which the unit is installed, and information for identifying the sensors assigned to the unit and facilities assigned to the unit; and a CPU programmed to act as: an identification section for identifying an internal unit being another unit installed in the target building and an external unit being another unit installed in a building other than the target building by the unit-specific information; an internal optimization section for reading an energy calculation expression of an internal unit identified by the identification section, selecting any one of operation conditions provided in the unit in advance, composing an energy calculation expression of an entire target building and performing simulation, and optimizing an energy balance of the target building; a related unit search section for searching for other units having an architectural condition related to the unit, the other units including a unit determined by the identification section as an external unit in addition to the other internal units; an internal and external optimization processing section for reading an operation condition corresponding to the unit from the other internal units and external units searched by the related unit search section, performing simulation using the read operation condition and the entire energy calculation expression, and optimizing an energy balance of the target building; and an operation section for operating the facilities assigned to the unit on the basis of the optimized operation conditions and the information from the assigned sensors, wherein the building is a collection of at least two buildings where energy management is integrally performed; wherein the target building is another collection of at least two buildings where energy management is integrally performed; wherein the building other than the target building is yet another collection of at least two buildings where energy management is integrally performed.