1. A method of network system including a first storage system, a second storage system coupled to the first storage system via a first network and a third storage system coupled to the second storage system via a second network, for storing files, comprising: providing a first logical unit in the first storage system as a first storage class, in which a first file of a first file system is stored in the first logical unit; providing a second logical unit in the second storage system as a second storage class, in which data of the first file is migrated from the first logical unit; providing a third logical unit in the third storage system as a third storage class, in which the data of the first file is migrated from the second logical unit; setting a lifecycle to the first file of the first file system, in order to be migrated from the first storage class to the second storage class, and from the second storage class to the third storage class; when the data of the first file is migrated from the first logical unit to the second logical unit based on the lifecycle of the first file, associating, by the first storage system, first identification information to a first file name managed by the first storage system, the first identification information corresponding to a first link destination of the data of the first file in the second logical unit; and when the data of the first file is migrated from the second logical unit to the third logical unit based on the lifecycle of the first file, associating, by the second storage system, second identification information to a second file name managed by the second storage system, without changing the first identification information associated to the first file name managed by the first storage system, the second identification information corresponding to a second link destination of the data of the first file in the third logical unit.