1. A demand forecasting computer system, comprising: 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 based on behavior patterns of the users to use the IT resources; an event characteristics model generation unit that calculates, using a computer processor, a variation over time in a usage state of the IT resources with respect to each user's behavior class based on the user's behavior classes and a log data generated at a time of a past event, calculates the number of the users moving among the user's behavior classes caused by the event by comparing classes generated before the starting of the event with classes generated after the ending of the event, and generates an event characteristics model representing the variation over time in a usage state of the IT resources with respect to each user's behavior and the number of users moving among the user's behaviour 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 for each user's behavior class 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 based on the predicted values of the variation over time and the number of the users moving that are calculated by the event influence calculation unit.