The present invention, in a computer system where multiple computers share multiple external storage devices, is to control the external storage devices of different control methods in a unified manner. A storage managing program, which manages the storage areas in the unified manner, includes a device control library providing control programs to control the various external storage devices, and a device information database identifying the external-storage-device controlling programs, and manages the multiple external storage devices of different control methods: Also, when a host computer releases an unnecessary storage area, a LU that no host computers use is deleted in the external storage device, and managed as an unused area can be created a new LU.