Abstract: An expert system building method wherein, in an expert system which executes an inference process according to a decision tree, at least one data unit or any combination of such data units of data forming an input/output information is made. Next, a data input/output method is specified on the basis of relationship between the at least one data unit and the input/output information. Thus, data possibly needed can be stored simultaneously. The expert system can execute an inference process according to the decision tree having the expert's knowledge described as they are with no relation to the data input, and the data can be simultaneously entered in units which are easy to understand by an end user with no relation to the process of the decision tree.