PURPOSE: To protect data in a medium in use by prohibiting the disconnection of a power source during executing an input/output processing operation even when a factor to disconnect the power source due to the turning off of a power switch or the voltage drop of the power source occurs, and furthermore, expediting a user to pay attention by issuing an alarm tone.

CONSTITUTION: Information during performing the input/output processing of data in a disk device, etc., is registered on a storage area by a storage area registration means 3 as a pre-processing before the execution of the input/output processing is performed. Furthermore, in the case of detecting the factor to disconnect the power source in a means 2 to detect the turning off of the power switch 1 or the voltage drop of the power source, a power source disconnection judging means 4 decides the continuance of the connection of the power source when registration information in the storage area registered previously shows the input/output processing under execution, and generates the alarm tone to the user by driving an alarm tone generating means 5. In such a way, the data in a disk medium in use can be prevented from being destroyed.