PURPOSE: To improve the reliability and to increase the starting speed at recovery of a commercial power supply for an electronic disk subsystem by saving the data contents to a data saving magnetic disk device by a battery power supply at discontinuation of the commercial power supply and at the same time holding the data contents of a data storage part.

CONSTITUTION: At supply of a main power supply, the main power supply is applied to a data storage part (b) and a main control part (a) together with a charging power supply applied to a battery (c) respectively. When the main power supply is inactive, the battery power (c) is supplied to the part (b) to hold the data contents of the part (b). At the same time, the data contents of the part (b) are saved into a data saving magnetic disk device (d). In such a constitution, the data contents can be assured double for improvement of the reliability. In addition, a system can be started at a high speed at recovery of the power supply.