A level-2 virtual machine is constructed under the control of a level-1 operating system (OS) operating on a real machine (level-1), and a level-3 virtual machine is constructed under the control of another operating system (OS) operating on the level-2 virtual machine. A level-3 virtual address generated in the level-3 virtual machine is translated to a level-2 virtual address, which is further translated to a level-1 virtual address. A third predetermined main storage address is added to the level-1 virtual address to generate a level-1 absolute address. The translated address is checked as to whether it is within a predetermined area on the main storage.