There is provided a product information supply method for supplying a user who desires to purchase a product with proper information about a related product that could be bought in combination with the product, so that the user is assisted in purchasing products. Registration of combination information to be supplied to the user is made with a database managed by a service provider server by a person who has bought the above product by means of a registration page so that a lot of combination information is accumulated in the database. The registered information includes not only information specifying a combinable product but also information about the effects of the combination and the ways of using products in combination. The database is searched in response to inquiry information from the user who makes reference to a page of products. Thus, corresponding combination information is extracted from the database and is sent to the user.