When a processor, which transits from a first mode that causes a guest operating system to operate to a second mode that causes a virtual machine monitor managing the guest operating system to operate, when previously set transition condition is satisfied, transits to the second mode, a determining unit determines a cause or the transition. When it is determined that an execution of a process related to a completion of writing the image information in an image storage unit on the guest operating system is the cause, a detecting unit detects an updated portion representing an unmatched portion of the image information between before and after writing.