PURPOSE: To possess jointly the same file device without interchanging a media between plural sets of OS by providing a memory area, an adder, a file accessing device and a processor in a joint possessing device of file.

CONSTITUTION: A file device 1 consists of a controlling information sector 10, the first group of segments 11 for OS that bisects whole remaining space and the second group of segments 12 for OS. A logical device 2 consists of a processor 20, a storage device 21, a file accessing device 22 and an adder 23. When the first OS runs on the logical device 2, the controlling information sector 10 for which physical position of inside of the file is determined beforehand is loaded to the storage device 21 by the file accessing device 22. When accessing each file for OS, accessing is made according to the result of addition of relative address in the OS of an aimed file and forefront address of data area for aimed OS in controlling information by the adder 23.