In a virtual machine system (VMS) capable of concurrently running at least one operating system (OS) under one real computer system and a control program (VMCP) for controlling the VMS, the object is to reduce the overhead produced for simulating VM I/Os by direct I/O execution. A VM information area of a real sub-channel control block has a status field in which a flag indicating that the sub-channel is dedicated or not is contained. When the flag is "0", it means that the sub-channel is dedicated to the VM and the sub-channel scheduling by the VMCP is not necessary. As a real interruption priority order is dedicated to a VM, only I/O interruption requests of the VM are queued into the real interruption request queue of that dedicated priority order, and the mixing of VMs in that real interruption priority order is avoided. When an interruption control mask of an interruption priority order of the OS on the VM is "0" indicating that the interruption is not acceptable by the VM, the interruption control mask of the corresponding dedicated real interruption priority order is also "0" and the hardware interruption does not take place. Accordingly, the interruption is retained by the hardware and the I/O interruption retention for the VM by the VMCP is avoided.