1. A demand forecasting apparatus comprising: a demand data storage which stores demand values of commodities in association with an identifier of each commodity; an accumulation trend calculator which calculates accumulation trend based on demand values stored in the demand data storage; a trend function calculator which calculates a trend function representing a demand trend based on the accumulation trend; a periodic-change determinate which determines whether there is a periodic change in the demand trend or not based on the difference between the demand trend and the trend function; a demand trend ratio calculator which calculates a demand trend ratio representing a ratio of the demand trend to the trend function; a periodic-change model calculator which calculates a periodic-change model by calculating monthly average of the calculated demand trend ratio as static value; a seasonal change forecaster which calculates a forecast value of seasonal change by combining the periodic-change model and the calculated trend function; wherein the periodic-change model calculator comprises: a logarithmic value calculator which calculates a natural logarithmic value of the demand trend ratio; and a periodic function fitter which performs fitting of the calculated natural logarithm of the demand trend ratio using a periodic function, and the periodic-change model calculator calculates the periodic-change model of a normal scale by converting the logarithm scale to a normal scale for the periodic function having undergone the fitting.