1. A stream data processing method for processing a stream which is a flow of time series data including plural data tuples to which each time stamp is applied and which are continuously incoming in the ascending order of time stamps, said method comprising the steps of: computing a first index rating a incoming data tuple of the stream according to index function algorithm utilizing the stream in predetermined period of time; associating the first index with the incoming data tuple; processing the incoming data tuple according to a registered query; acquiring a second index based upon the first index associated with the incoming and preceding date tuples; and outputting the processed data tuple and the acquired second index as an index rating the processed data tuple.