A structuring unit generates a combination pattern of the attribute values by combining an attribute value of the assessment target data with an attribute value indicating a unspecified value. A reconstructing unit searches data matching the combination pattern from the accumulation data, and generates subset data. An index setting unit sets an index indicating significance of relationship between the generated subset data with assessment data. An assessing unit selects subset data that becomes a basis of assessment, based on the set index, and performs an assessment on a prediction result based on the selected subset data.