Provided is a technique to enhance security of a computer in a multi-OS operating environment. A memory image of a primary OS is in an active state, and a memory image of a secondary OS is in a non-active state. When switching is performed from the primary OS to the secondary OS, a runtime image is created, and a digest P is calculated and encrypted (C). When switching is performed from the secondary OS to the primary OS, a digest of the stopped runtime image is calculated. Only when comparison between the decrypted digest P and the calculated digest shows agreement, is switching of the primary OS allowed.