An identification information centralizing method capable of receiving services provided by a plurality of corporations by using a single item of identification information. Centralized ID information (162) for identifying a customer is assigned by a centralizing corporation. Individual ID information (168) which is uniquely assigned to identify the customer by a plurality of corporations which provide services to a pre-registered customer is registered in a database (160) in such a manner as to be associated with the centralized ID information (162), so that provision of services by a plurality of corporations is made possible by using the single item of centralized ID information (162).