A credibility determining system enabling highly accurate credibility determination of given information includes causal relation knowledge DB 90 and search scope constraint DB 88. Causal relation knowledge each includes a combination of cause and result parts, and time and position constraints associated with the causal relation. The credibility determining system further includes a query generating unit 92 retrieving a causal relation matching the input information and based on the time and position constraints stored in search scope constraint DB 88 in association with the retrieved causal relation, generating a query for retrieving remarks from mini-blog text DB 84, a text search unit 96 searching for related remarks from mini-blog text DB 84 using the query, and a display candidate selecting unit 100 classifying the searched remarks to expressions interpreted as causes or to results, determining credibility of the input information based on the classification result and outputting the determination result.