An energy-saving effect calculator which includes a unit for determining a standard value of each of a plurality of patterns created from a past operation mode, a past demand data, and past operation data of a boiler; and a comparing unit for creating patterns of an operation mode, demand data, and operation data at a present time, and comparing a value of the pattern with the standard value, wherein at least one of a reduction in energy cost and a reduction in CO2 is calculated based on a comparison result of the comparing unit.