1. A virtualization program that causes a computer to execute a process, comprising: creating a stack region for storing function call information with a LIFO (last in first out) structure in the order of functions to be called sequentially by each of tasks generated on a target OS, or a subject of simulation, a register preserving region that stores the register contents at the last interruption of execution of each of the tasks on a task by task basis as most updated register information and a task state storing region (task control block: TCB) that stores the state information showing the state of each of the tasks on a task by task basis at each time of starting each of the tasks; storing the register contents at the time of calling the function at least immediately preceding the task called last time by the task being executed in the register preserving region as the most updated register information and also storing the state information showing the state of the task being executed in the task state storing region; and switching from the task being executed to the standing by task according to the most updated register information and the state information on the standing by task to be executed next for a native code simulation of the target OS adapted to execute a multitask process.