PURPOSE: To prevent contents in an external storage device from being read and being written by a method wherein a part of an optional form having a size by which it can be freely penetrated is fitted to a hole of a plate like external storage device having the hole which has preliminarily passed through the storage device, and is formed so that a component can not be removed from the external storage device unless the components is broken after fitting.

CONSTITUTION: One or a plurality of optionally formed components 2 having a size by which they can be freely penetrated are fitted to a hole of a plate like external storage 1 having a hole which has previously passed through the storage. At that time, the component 2 are fixed so as not to be removed from the plate like external storage device 1 with a pawl 3 provided to the components 2. After fitting the components 2, the components 2 becomes incapable of being removed from the plate like external storage device 1 unless an optional position of the components 2 is broken. The external storage device 1 fitted with the components 2 can not be accurately fitted to a device which reads and writes contents since the fitted components 2 are a hindrance thereto, and the contents become incapable of being read and being written.