A method of tier management of data comprises performing a tier migration log information setup process which includes selecting an area specified by a virtual volume address and a logical volume address; determining a destination tier for the area based on a number of accesses to the area; and updating a tier migration log information by inputting the determined destination tier and a time; and performing a process using the tier migration log information to determine whether to migrate a specific area which includes loading a tier migration log from the tier migration log information by selecting a specific time; checking if a current tier of the specific area equals a destination tier specified by the tier migration log; and if the current tier is not equal to the destination tier, migrating the specific area to the destination tier.