To provide an authenticating system and authenticating method capable of performing secure authentication while using a personal portable terminal such as a portable telephone set at the same time.

When a session is established between a user terminal 203 and a network server 204, a session number is sent. A user 208 inputs it to his/her own portable telephone set 209 to transmit it to a portable telephone company server 206. The portable telephone company server 206 transmits the session number and a telephone number read at the time of call connection to the network server 204 where it is going to perform authentication. When the telephone number specific to the portable telephone set 209 of the user 208 coincides with the session number, the network server 204 succeeds in authenticating the session.

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