Abstract: A cause inferring device which infers causes with improved accuracy and efficiency. A phenomenon item (F) other than phenomenon items (A, B and C) for which frequency-of-occurrence data are already inputted is selected based on the degree of association data regarding cause items (2, 3 and 4) having the highest certainty among cause items (1-5) whose certainties are calculated. The frequency-of-occurrence data of the selected phenomenon item (F) and already inputted frequency-of-occurrence data are inputted again and the certainties of the cause items are computed. By repeating such selection and arithmetic processing at least once, causes are narrowed down as a final inference.