A computer system includes a first volume that is read and written from a first computer and in which write data is written, a second volume storing journal data in the first volume, the journal data delimited at each predetermined point, a third volume as a virtual volume, a unit that creates, when a backup instruction for the first volume at a predetermined point is received, the third volume from which a second computer reads the journal data and in which the second computer writes the journal data, a unit that maps the journal data to the third volume, and a unit that transfers the write data to a storage device via the second computer or transfers, through the third volume to which the journal data is mapped, the journal data to the storage device via the second computer and backs up the write data and the journal data.