The present invention is for shortening the time required to discover a phenomenon related to a detected event. Reverse reproduction is performed in data stream processing. Specifically, a computer system 3 processes multiple pieces of input data 141 in accordance with times in chronological order denoted by the timestamps 142 within the input data 141, beginning from input data 141 including a timestamp (hereinafter, latest reference stamp) that denotes the newest time from among the timestamps denoting times prior to a reference time (for example, an event detection time). A method for creating output data 171 including the result of this processing and output timing are controlled on the basis of the lifetime of the input data 141, which is specified by a query 131. In accordance with this, the output data is properly outputted using reverse reproduction.