The external storage device according to the present invention which uses a non-volatile semiconductor memory such as a flash memory is provided with plural areas which store user data, and restricts access to the user data from a host computer and also dynamically changes an area an access to which is to be restricted. Accordingly, the ease of use of the external storage device for the host computer is improved. Specifically, the interior of the flash memory is divided into a normal area not protected by a password or the like and a protected area protected by a password or the like. A microprocessor controls accesses to the normal area and the protected area in accordance with a command from the host computer. In addition, the host computer can access the protected area after passing through authentication using a password. Moreover, the host computer changes information indicative of the location of the protected area, thereby dynamically changing the protection area.