The file sharing system of the present invention selects a suitable data migration method that corresponds with the usage status and transfers data efficiently. A virtual volume and a migration source volume which is the migration target volume are associated by means of an external connection control section. When an instruction for volume migration is issued, the usage percentage judgment section compares the usage percentage of the migration source volume with a designated threshold value. When the usage percentage is equal to or more than the threshold value, volume copying is selected. The volume copy section copies data at the block level from the virtual volume to the migration destination volume. When the usage percentage is less than the threshold value, file copying is selected. The file copy section reads files stored in the migration source volume via the host and writes the files to the migration destination volume.