On a basis of information obtained by reading information for specifying articles carried by a user from identification tags attached to the articles by means of a sensor, a list of the articles carried by the user at the time point is formed. The list is stored every time the list is formed, and the event that the user has lost the article and the lost article are specified on the basis of two or more lists. Upon specifying the lost article, the user is notified of information concerning the lost article.