To provide a home building design plan meeting to a customer's desire to the customer who plans home building with an easy procedure.

When the server device of an intermediary agent receives a design plan forming request from the customer side who plans the home building, a questionnair is transmitted from the server device to the customer to collect desire information related to housing, and asks a design plan formation to a prescribed builder based on the desire information. The design plan formed by the builder is transmitted to a customer-side terminal device. When a correction is requested from the customer side, the correction is asked to the builder. When the customer finally agrees it, estimations are obtained from a plurality of builders based on the design plan, and an optimum builder is selected from the estimation result. These procedures are almost automatically performed through the Internet.