A contactless IC chip is mounted or adhered on a certificate and used in a service system having an interrogator and a computer system. The interrogator and the computer system are interconnected via a network such as the Internet. The IC chip is used as an electronic tag and the interrogator receives first information by reading information printed on the certificate including a digital signature and by receiving second information from the electronic tag through an antenna of the interrogator. Then, the first information from the certificate and the digital signature are used by the computer to calculate third information that is compared with the second information received from the electronic tag. If the comparison results in a match, then the certificate is deemed authentic.