It is an object of the present invention is to provide a disk unit capable of restricting access to data stored in the recording medium depending on users. The user enters a password into a host computer 100. This password is transferred to a password verification section 20. Then, the password verification section 20 checks whether the password coincides with any one of authentication passwords 11 through 13 stored in an authentication password storage section 10. If the password coincides with any one of the passwords, the password verification section 20 supplies an authentication signal to an operation mode control section 30. The operation mode control section 30 supplies an operation mode signal to a controller 50 by receiving the authentication signal. The controller 50 controls execution of the operation mode according to the operation mode signal.