A system for preventing fraud of certification information performs a processing step of processing web page data, a first comparing step of comparing the web page data processed in the processing step with web page data stored in a web page storage unit in association with URL of the data, a second comparing step of comparing URLs of the web page data similar to each other when the first comparing step determines that the web page data are similar to each other, and an address attention message adding step of adding an address attention message to the web page data processed in the processing step when the second comparing step determines that the URLs differ. Therefore, a user who is to access a web site can determine whether or not this web site is authentic, and certification information fraud can be prevented beforehand.