A virtualization technique for using the hardware resources of a computer regardless of the physical configuration of the computer is known in the art. Using virtual servers, implemented by the virtualization technique, a single physical server may be logically partitioned and used as if the physical server is a plurality of servers. For example, different operating systems (OSs) and applications can run on a plurality of respective virtual servers that operate on a single physical server Typically, the use of, for example, a server device may be charged. In some known charging methods, the power consumption of a used device is measured, and a charge is made on the basis of the measured power consumption In a case where the use of a physical server is charged, a charge may be made on the basis of the power consumption. However, in a case where the use of a virtual server is charged, simple charging based on the power consumption of each device is a disadvantage. Specifically, when a plurality of virtual servers operate on a single physical server, since the hardware resources of the physical server are shared by the virtual servers, the amount of charge to the use of each of the virtual servers is affected by the operations of the other virtual servers operating on the same physical server. Moreover, in a case where the type of a used physical server (hardware resources) is changed, even when a virtual server achieves the same performance and provides the same level of service, the power consumption may vary, and thus the amount of charge may change.