PURPOSE: To prevent the loss and breakdown of data due to an unfinished procedure by executing the processing procedure corresponding to a break request of a power supply delivered from a follower processor and breaking the power supply after all procedures are through.

CONSTITUTION: A host device 11 accepts a power supply break request and starts a power supply break processor 14 to transfer the identification information corresponding to the requester as well as the type of the power supply break request and the device information showing a device to which the power supply is cut off. The call information corresponding to the identification information is extracted out of a file storing the procedure information and given to the processor 14. This call information is transferred to the device 11. The device 11 executes the processing procedures that should be finished before break of power supply in accordance with the call information. When these processing procedures are through, the information is delivered to show the end of the processing procedures. Receiving this information, the processor 14 delivers a power supply break control signal to a power supply control line corresponding to the device that is designated by the device information. Thus a switch is turned off and the power supply is cut off.