A thread status managing unit organizes a plurality of threads into groups and manages the status of the thread groups. A ready queue queues thread groups in a ready state or a running state in the order of priority and, within the same priority level, in the FIFO order. An assignment list generating unit sequentially retrieves the thread groups from the ready queue. The assignment list appends a retrieved thread group to a thread assignment list only when all threads belonging to the retrieved thread group are assignable to the respective processors at the same time. A thread assigning unit assigns all threads belonging to the thread groups stored in the thread assignment list to the respective processors.