an comprises an external storage device and an external storage device unit , wherein an external storage device main includes , a thin type external storage device module formed into a one - side sealed package from a storage medium series element containing at least one non - volatile semiconductor memory device , and a flat type external connection terminal connected to an input / output terminal of the storage medium series element and led and exposed to a backside of the external storage device module , and an external storage device unit includes , an external storage device main detachable section for engaging , insertedly attaching and detaching the external storage device main , an resilient contact electrically connecting to the external connection terminal of the external storage device main to be mounted , and at least a part of circuit function for driving and controlling the storage medium series element .