When a user process issues an input/output request in a process performed in response to the input/output request generated by the user process, the input/output request is queued in the input/output request list which is a queue in a list format on the user space side. On the kernel side, when a input/output request in the input/output request list is processed, the status is changed into ‘processed’, and a list element whose status indicates ‘processed’ is removed from the request list on the user space side. On the kernel side, a thread for performing a process in response to an input/output request is divided into a plurality of threads, and the CPU is released to another thread before completion of the process.