To make sellable a product by making the body design, exterior and material, etc., of the product match with preferences of a purchaser at the time of selling the product such as a personal computer to the purchaser through a network such as the Internet.

The center device of an electronic commercial transaction system is provided with a material selection information presenting part 45 presenting information for selecting the material of a body to a purchaser side, a shape design tool presenting part 47 presenting a design tool for supporting the design of the shape of the body to the purchaser side, a plotting tool presenting part 49 presenting a plotting tool for supporting the design of the appearance of the body to the purchaser side, a designed result presenting part 51 obtaining a designed result of the product by integrating a material selected result, design information of the body shape and the design information of the body appearance from the purchaser side and presenting the designed result to the purchaser side, and a selling condition presenting part 52 calculating a price and delivery time of the product and presenting them to the purchaser side.