A file server including: a first interface coupled to a client computer which manages a client side user identifier used by the client computer to identify a client computer user; a second interface coupled to a first storage storing first file system data and a first file system side user identifier used by the first file system to identify the client computer user, and a second storage storing second file system data and a second file system side user identifier used by the second file system to identify the client computer user; a processor which receives a client computer's first access request to the first file system, obtains a first file system identifier which identifies the first file system and the first file system side user identifier, and translates the first file system side user identifier to a first client side user identifier using the first file system identifier.