1. A method for calculating a confidence degree as a degree of certainty in reasoning with use of similarities between cases of a record type including at least two fields, comprising the steps of: setting at least one input field for use in calculation of the similarities between the cases and at least one output field to be an object of the calculation of a reasoning result; inputting parameters relating to the similarity calculation including an allowable value of a reasoning error; inputting a new case having at least one output field having an unknown value; calculating similarities between said new case and previous cases already stored for each of the previous cases; generating a distribution of the output fields by referring to values of the output fields of the previous cases in a decreasing order of said similarities; comparing said distribution with said allowable value to calculate a confidence degree in the reasoning; calculating a reasoning result of the output field value of the new case with use of the output field values of the previous cases and said calculated similarities; and outputting said reasoning result and said confidence degree.