invention in other words a quantity of usage of a cpu is calculated by using a system service provided from ms windows in windows a vmm virtual machine manager service usable in an embodiment of a vdd virtual device driver is provided in other words a service related to a timer and threads is used in the vmm service and preferably operating at a ring 0 level in general a program operated in the system is classified as a thread and the thread is a minimum unit of execution herein the execution means the cpu is used as described in fig1 a variable sum storing the sum of the execution time of all threads excluding the system thread is initialized as in step s 10 a handle value hd 1 is then read as a pointer of the system thread in step s 11 and the next thread handle value hd 2 is read in step s 12 after reading the handle value hd 1 of the system thread and handle value hd 2 of the next thread they are compared in step s 13 to determine whether the read values are the same in other words it is judged whether the all thread values have been read by comparing the handle value read most recently with the handle value of the system thread.