A changer apparatus capable of providing security protection. Tape cassettes are selected from a plurality of compartments constituting a cassette rack, and are loaded into drives (102-1) through (102-P) which write and read data to and from the cassettes. A nonvolatile memory (114) stores information about security protection locked on elements such as the compartments of the medium rack and the drives. When security protection is to be locked, a controller (110) admits an externally input security protection locking command, and writes accordingly to the memory (114) security protection information representing, for example, host IDs, password numbers, security levels, and a group of elements of which security is to be protected. When security protection is to be unlocked, the controller (110) admits an externally input security protection unlocking command, and erases the security protection information accordingly from the memory (114). When executing a command from a host computer, the controller (110) references the security protection information in the memory (114) for control over command execution, whereby security protection is ensured.