A demand forecasting system and a demand forecasting method capable of forecasting demands for IT resources in a data center accurately are provided. The demand forecasting system includes: a classification unit that classifies a plurality of users who use IT resources in a data center into a plurality of user's behavior classes; an event characteristics model generation unit that generates an event characteristics model representing a variation over time in a usage state of the IT resources caused by an event and the number of the users moving among the user's behavior classes caused by the event; an event influence calculation unit that calculates a predicted value of the variation over time in a usage state of the IT resources and a predicted value of the number of the users moving among the user's behavior classes that will be caused by an event scheduled to be held in the future using the event characteristics model; and a demand forecasting unit that calculates a variation in load on the IT resources.