A certifying method comprising: a request step (SA1, m1) for transmitting a connection request by a communication device to start communication with another communication device; a transmission step (SB4, m2) by a relay device for transmitting time information corresponding to the clocking present time of said relay device to said communication device when said communication device starts communication with said another communication device through said relay device, wherein the transmission step is performed subsequently to said request step (SA1, m1) and prior to commencing communication with said another communication device; a correction step by said communication device for correcting the clocking present time of said communication device on the basis of said time information transmitted in said transmission step and received by said communication device; a relay step (SC3, m6) by said relay device for relaying certificate information with a certificate validity period, said certificate information being transmitted from said other communication device to said communication device, and certifying the authentication of said other communication device or said other communication device user; a judgment step (SA8, SA9) by said communication device for judging whether the present time is within the validity period specified in said certificate information relayed in said relay step and received on the basis of said present time of the concerned communication device corrected in said correction step; and a certificate step for judging the authentication of said other communication device or said other communication device user by said communication device through the judgment result in said judgment step and said certificate information. a request step (SA1, m1) for transmitting a connection request by a communication device to start communication with another communication device; a transmission step (SB4, m2) by a relay device for transmitting time information corresponding to the clocking present time of said relay device to said communication device when said communication device starts communication with said another communication device through said relay device, wherein the transmission step is performed subsequently to said request step (SA1, m1) and prior to commencing communication with said another communication device; a correction step by said communication device for correcting the clocking present time of said communication device on the basis of said time information transmitted in said transmission step and received by said communication device; a relay step (SC3, m6) by said relay device for relaying certificate information with a certificate validity period, said certificate information being transmitted from said other communication device to said communication device, and certifying the authentication of said other communication device or said other communication device user; a judgment step (SA8, SA9) by said communication device for judging whether the present time is within the validity period specified in said certificate information relayed in said relay step and received on the basis of said present time of the concerned communication device corrected in said correction step; and a certificate step for judging the authentication of said other communication device or said other communication device user by said communication device through the judgment result in said judgment step and said certificate information.