In a computer system including a disk subsystem having channel interface compatible to a count-key-data format and a SCSI interface compatible to a fixed length data format, the disk subsystem is connected to a CPU controlled by an open system operating system through the SCSI interface, and connected to an other CPU controlled by a main frame operating system through the channel interface. The CPU is provided with a CKD record access library and a VSAM access library which accesses in a FBA format the VSAM record stored by the other CPU in a CKD format in the disk subsystem and allows the access by an application program of the CPU as a VSAM record based on VSAM control information.