AUTHENTICATION METHOD, COMMUNICATION APPARATUS, AND RELAY APPARATUS

Included are Gateway server GWS which clocks the precise present time, and portable phone MS which performs a packet communication through gateway server GWS and IP server W. Portable phone MS acquires the time information from gateway server GWS, at the time of starting communication with IP server W and corrects, on the basis of this time information, the clocking present time of its own portable phone MS so that it is more precise. In addition, portable phone MS decodes a public key certificate (an electronic certificate issued by Certificate Office C for the public key certificate of IP server W) using the public key of Certificate Office C. Then the portable phone MS judges whether it is within the validity period specified in the public key certificate, using the corrected present time of its own portable phone MS.