Apparatus for managing key in afile security system and method for managing security key

A file security system uses a security class set by an access control module. The file security system includes a disk, a kernel memory and an encryption file system. The disk includes a key file in which an encryption key corresponding to the security class is stored and a file encoded by the encryption key. The encryption key stored in the disk is loaded into the kernel memory when the file security system starts operating. The encryption file system extracts an encryption key corresponding to a security class of a file that a user intends to read or store; decodes or encodes the file by using the extracted encryption key; and then provides the decoded file to the user or stores the encoded file in the disk.