PURPOSE: To retrieve a route at low cost by retrieving the route for one solution by means of repeating questions and answers with a user through the use of tree structure showing the route for the solution by means of the questions and answers.

CONSTITUTION: When the user selects 'undefinition', it is assumed that he cannot answer any displayed questions, a processing in which all the routes following the elements of a question list are set to be candidates and the node of the candidate routes are set to be the new question list is executed. When the user selects one of the displayed questions, a processing in which the route from the node to the terminal node of the route decided by that time is calculated and the calculated route is shown to the user is executed. When the user selects the route, it is assumed that the route to that node is decided and a processing in which the preceding node of the decided route is set to be the new question list is executed. Above processing is repeated until it reaches the bottom.