In a storage device (30), a dump process can be conducted without any intervention of a CPU, and dump objective data at a dump start point can be completely dumped even when a data update is conducted during a period of time from the dump start point to a dump end point. A disk device with dump function (30) transfers data from a data not-updated region of a magnetic disk (100) to a disk device (31) as a dump destination in a predetermined sequence. In a case where a data update request is received during a period of time from the dump start point to the dump end point, when an area of the write data is other than the data not-updated region, the write data is written on the magnetic disk (100). When the area is in the data not-updated region, a before image block thereof is saved in the buffer (70) and then the write data is written on the magnetic disk (100). Thereafter, at an earlier one of the opportunity to transfer the pertinent data to the dump destination disk device (31) when the quantity of data saved in the buffer (70) exceeds a predetermined value or the opportunity to transfer the data thereto when an update request for the data is not done, the data saved in the buffer (70) is transferred to the dump destination disk device (31).