Data stored in a plurality of snapshots that can be updated is efficiently updated.A computer system includes a computer that provides a plurality of storage areas and a storage device that stores data read and written in the storage areas. The storage device provides a first volume that stores data shared by the storage areas respectively and a second volume that stores data held in the storage areas respectively. The computer stores storage area management information and a storing flag that shows a storing part of updated data and sets the storing flag in accordance with a request. When the computer receives the update of the data stored in the first volume and the storing flag is set to the second volume, if a second area corresponding to a first area is not allocated to the second volume, the computer updates a second area newly allocated to the second volume to store the location information of the newly allocated second area in the storage area management information correspondingly to the first area.