PURPOSE: To make only unsaved data which is under a prescribed number save when a power source is disconnected by stopping an access for a while to save in order that a host device does not execute a write in the data if the write of the data is executed in a semiconductor memory by the prescribed number.

CONSTITUTION: When a data save/restoration mechanism 12 scans on a control storage 11 and a flag which shows that the data is written in the semiconductor memory 5 reaches the prescribed number, the data is made to save from the semiconductor 5 to a nonvolatile storage device by the prescribed number. Therefore at the time of receiving a power source disconnection signal from a power source disconnection/throw-in entry control circuit 6, it is needless that all the contents of the semiconductor memory 5 is made to save. At this time it is necessary only that only unsaved data, which is naturally under the prescribed number, is made to save, so that the power source can be disconnected in a short saving time.