A virtual machine system which includes a plurality of virtual machines by using a computer system of a multi-processor configuration having a plurality of real instruction processors and a real main storage which is divided into a plurality of storage regions to be allocated to the virtual machines, respectively. Each of the virtual machines is so organized as not to make access to the regions allocated to the other virtual machines. When one and the same virtual machine includes a plurality of real instruction processors, invalidation of entry of a buffer storage of another real instruction processor as conditioned by execution of a predetermined instruction by a real instruction processor is performed only for the other real instruction processor assigned to the same virtual machine as the real instruction processor and is inhibited from affecting the real instruction processors assigned to the other virtual machines.