To obtain a decision result equivalent to that obtained when a human being describes the loss value by securing the correspondence between the proposition of an action pair and the small real number value when the similarity is high to the optimum action pair, leading the expected loss value of an action based on the loss value and the posterior probability value, and selecting an action of the least expected loss value.

A loss data processing part 2 sends a pair of propositions to a similarity calculation part 3 to secure the correspondence between the propositions and the similarity of ≥0.0 and ≤1.0 real number value. A loss value calculation part 4 secures the correspondence between an optional action pair and the loss value of ≥0.0 and ≤1.0 real number value. At the same time, the correspondence is secured between the optimum action pair and 0.0. Then the correspondence is secured between the loss value and the small value when the similarity is high to the proposition of the optimum action pair. A probability calculation part 6 leads the posterior probability value of each proposition from a fact and the probability data to output it to an expected loss value calculation part 7, and the expected loss value of each action is led from the posterior probability value and the loss data given from the part 2. Then an action decision part 8 extracts an action of the least expected loss value.