A hypervisor is operated while an I/O device is not controlled by the hypervisor performs a process operation. A result of the process of the I/O device which is being concurrently operated is recorded to be capable of being reflected to the operation of the hypervisor. Therefore, a hypervisor which assigns a computer including an I/O control device controlling an I/O device, a main storage device, and a CPU as resources to each of one or more logical partitions (LPAR) and performs control, and a logging circuit recording data transmission from the I/O device to the main storage device, which is uncontrolled by the hypervisor, in response to an instruction from the hypervisor, are provided. It is possible to shorten a halt time period of an LPAR.