An online data migration method is provided for transferring data from a computer and an existing storage system using the conventional interface such as SCSI to a new storage system using SAN. The present method comprises the steps of disconnecting the connection between the computer and the first storage system with first interface protocol, connecting the computer to a switch connected to a second storage system with a second interface protocol through first protocol converter having protocol converting facility, connecting the switch to the first storage system through second protocol converter, and migrating data in the first storage system into the second storage system via the switch.