1. A data processing device comprising: a data input unit configured to input test data in which a correct hypothesis is established in a learning mode for each of predetermined inputs, and inputs subject data for a hypothesis search in a search mode for each of the inputs; a feature amount extraction unit, implemented by at least one hardware processor, which is configured to analyze the input test data and the input subject data and extracts each feature amount; a hypothesis measure calculation unit configured to calculate a plurality of pruning measures for each of a plurality of hypotheses of the test data and the subject data using the extracted feature amount; a data plotting unit configured to plot the plurality of hypotheses of the input test data on a threshold space specified by the plurality of pruning measures, corresponding to each of the calculated pruning measures; an isopycnic surface setting unit configured to set a plurality of isopycnic surfaces on the threshold space, corresponding to densities of the plurality of plotted hypotheses; a threshold curved surface generation unit configured to generate, on the threshold space, a threshold curved surface made of the plurality of pruning thresholds in which a decrease in at least one of the plurality of pruning measures causes an increase in at least one thereof, using a portion of one surface selected from the plurality of isopycnic surfaces as a part; a hypothesis curved surface generation unit configured to generate a hypothesis curved surface made of the plurality of hypotheses of the subject data on the threshold space, corresponding to each of the calculated pruning measures; and a hypothesis pruning unit that configured to prune the plurality of hypotheses of the subject data, using a position at which the generated hypothesis curved surface intersects the threshold curved surface as the pruning threshold.