Data migration is efficiently executed with optional timing. A file server outputs an instruction to a CPU of a storage apparatus to migrate a file belonging to Tier 1 to Tier 2 when, based on policy information, a condition for migrating the file belonging to Tier 1 to Tier 2 is fulfilled. Based on the instruction from the file server, the CPU copies the data of migration target files in Tier 1 from a page to a Tier 1 virtual page, copies the data which has been copied to the Tier 1 virtual page 114 to a Tier 2 virtual page, copies the data which has been copied to the Tier 2 virtual page to any page in Tier 2, and then overwrites and releases the Tier 1 and Tier 2 virtual pages, and migrates files belonging to Tier 1 to Tier 2.