A unit: (a) sets event largest possible values used for limitation of the number of tasks and/or the size of areas in a buffer; (b) generates a task without reaching the event largest possible values when receiving an event, and reserves an area in the buffer for the task; (c) determines whether another event has been received before process of the task is completed; (d) deletes the task and releases the area if another new event has not been received; (e1) if it has been received and both the number of tasks and the size of the areas do not reach the event largest possible values, generates a new task and reserves another area in the buffer; and (e2) if it has been received and any of them reaches any of the event largest possible values, reuses the task and the area for the new event.