A processor of a mainframe host is provided with a variable length/fixed length format conversion function, and furthermore, provided with a function capable of connecting with a disk array provided outside a frame of the mainframe host by a fixed length interface. As a result, data to which the mainframe host, a UNIX server, and a PC server separately access can be commonly stored into the disk array equipped with the fixed length format interface. An interface for connecting a mainframe unit to an open system, is made identical to another interface for connecting a disk array which commonly stores thereinto data accessed by, for example, a UNIX server and a PC server, to both the mainframe host and the open system. As a result, a management step number of the computer system can be reduced, and the computer system can be easily utilized.