PURPOSE: To provide an ID number collating method and its device in an information terminal equipment to be connected to an IC card and an ID number input device.

CONSTITUTION: The ID number input device 120 computes a random number 130 formed by an IC card 100 and obtained through an information processing terminal 110 and an inputted ID number 136 by an arithmetic means 123, ciphers the computed result 137 by a ciphering means 122 and transmits the ciphered result to the terminal 110. Similarly to the device 120, the IC card 100 computes the random number 130 and a previously stored ID number 132 by an arithmetic means 106, and ciphers the result 133 by a ciphering means 104. The cipher data 134 are collated with cipher data obtained through the terminal 110 and the collated result is returned to the terminal 110.