A statistics system stores inputted statistical target information into storage. When each unit time period defined in a statistical calculation rule expires, the statistics system reads all pieces of statistical target information whose occurrence times are contained in the unit time period from storage, executes statistical calculation, and registers the statistical result to the statistical information in storage. When statistical target information is inputted, the statistics system sets a re-calculation flag to a unit time period including the occurrence time shown by the statistical target information among the respective unit time periods whose statistical results are registered in the statistical information. The statistics system reads all pieces of the statistical target information whose occurrence times are contained in the unit time period from storage for each of the flagged unit time periods, re-executing the statistical calculation, and overwrites the statistical result onto the information in storage.
