An input tuple including a data time stamp is assigned by a data source; a pending tuple in which a system time stamp at the time the input tuple was received is assigned to the input tuple; and the pending tuple is stored in a queue, one queue for each data source. The queues are sorted in the order of the data time stamps of the pending tuples at the head of each queue; and if, in the queue at the head of which is stored the pending tuple having the smallest data time stamp value, the value of the current system time stamp is greater than the value of a processing pending period added to the system time stamp of the pending tuple at the head of the queue, then the input tuple is acquired from the pending tuple at the head of the queue for an input stream.