To prevent a third party's improper use by providing a certifying number input instructing means urging the input of a certifying number corresponding to an optional random number code among plural kinds of certifying numbers registered by a user by means of a sound instruction.

The user inputs a certifying number corresponding to a previously registered numeral in accordance with the instructions of second and third announces from a push button. An exchange for a station 2 transmits a credit card registering number and the certifying number received from a public telephone set 1 of the user to an on-line certifying processing computer 4. The computer 4 certify-processes based on these of the credit card registering number and the certifying number. When the certifying processing by the computer 4 is certified, information of the result is informed to the exchange for the station 3. The exchange for the station 3 receives the processing result from the computer 4, checks whether the certifying is YES or not and at the time of YES, urges the input of the opposite telephone number to execute normal telephone exchange processing.