1. A demand forecasting method that forecasts a demand for a commodity using a demand data storage unit recording a demand trend comprising a time-sequential trend of demand values of said commodity, and a management computer, said management computer including: a trend-function calculating stage that calculates a trend function based on said demand trend recorded in said demand data storage unit; a demand-trend ratio calculating stage that calculates a demand trend ratio which is a ratio of said demand trend to said trend function; a periodic-model calculating stage that calculates a periodic model which approximates said demand trend ratio; and a demand-forecast calculating stage that calculates a demand forecast value of said commodity by calculating a demand forecasting model by multiplication of said periodic model by said trend function.