A storage system coupled to a host computer, comprising: a plurality of disk devices; a plurality of nonvolatile memories, provided as flash, MRAM or PRAM memories, each of the plurality of nonvolatile memories includes a plurality of nonvolatile memory modules and a memory controller that controls access to the plurality of memory modules; and a processor that is connected to the plurality of disk devices and to the plurality of nonvolatile memories; wherein the processor is configured: to assign a first storage area of the plurality of nonvolatile memories as a cache memory area in which write data is temporarily stored; and wherein the processor is configured: to provide a plurality of expansion volumes each identified with a respective logical unit number to the host computer; to create one or more pool volumes configured by the third storage area of the disk devices and by a second storage area of the plurality of nonvolatile memories, which is different from the first storage area, wherein in response to write requests from the host computer to a first expansion volume of the expansion volumes storage areas of the one or more pool volumes are allocatable to the first expansion volume; and to allocate by, in response to a write request from the host computer to store write data in the first expansion volume of the expansion volumes in case of a physical storage area not having been assigned to the write request target, the processor selecting and assigning a suitable storage area of a pool volume of the one or more pool volumes to the first expansion volume, whereby the write data is ultimately stored in the second or third storage area. a plurality of disk devices; a plurality of nonvolatile memories, provided as flash, MRAM or PRAM memories, each of the plurality of nonvolatile memories includes a plurality of nonvolatile memory modules and a memory controller that controls access to the plurality of memory modules; and a processor that is connected to the plurality of disk devices and to the plurality of nonvolatile memories; wherein the processor is configured: to assign a first storage area of the plurality of nonvolatile memories as a cache memory area in which write data is temporarily stored; and wherein the processor is configured: to provide a plurality of expansion volumes each identified with a respective logical unit number to the host computer; to create one or more pool volumes configured by the third storage area of the disk devices and by a second storage area of the plurality of nonvolatile memories, which is different from the first storage area, wherein in response to write requests from the host computer to a first expansion volume of the expansion volumes storage areas of the one or more pool volumes are allocatable to the first expansion volume; and to allocate by, in response to a write request from the host computer to store write data in the first expansion volume of the expansion volumes in case of a physical storage area not having been assigned to the write request target, the processor selecting and assigning a suitable storage area of a pool volume of the one or more pool volumes to the first expansion volume, whereby the write data is ultimately stored in the second or third storage area. to assign a first storage area of the plurality of nonvolatile memories as a cache memory area in which write data is temporarily stored; and wherein the processor is configured: to provide a plurality of expansion volumes each identified with a respective logical unit number to the host computer; to create one or more pool volumes configured by the third storage area of the disk devices and by a second storage area of the plurality of nonvolatile memories, which is different from the first storage area, wherein in response to write requests from the host computer to a first expansion volume of the expansion volumes storage areas of the one or more pool volumes are allocatable to the first expansion volume; and to allocate by, in response to a write request from the host computer to store write data in the first expansion volume of the expansion volumes in case of a physical storage area not having been assigned to the write request target, the processor selecting and assigning a suitable storage area of a pool volume of the one or more pool volumes to the first expansion volume, whereby the write data is ultimately stored in the second or third storage area. to provide a plurality of expansion volumes each identified with a respective logical unit number to the host computer; to create one or more pool volumes configured by the third storage area of the disk devices and by a second storage area of the plurality of nonvolatile memories, which is different from the first storage area, wherein in response to write requests from the host computer to a first expansion volume of the expansion volumes storage areas of the one or more pool volumes are allocatable to the first expansion volume; and to allocate by, in response to a write request from the host computer to store write data in the first expansion volume of the expansion volumes in case of a physical storage area not having been assigned to the write request target, the processor selecting and assigning a suitable storage area of a pool volume of the one or more pool volumes to the first expansion volume, whereby the write data is ultimately stored in the second or third storage area.