Abstract: A terminal certification system with a high security, which is capable of detecting and eliminating an illegal access by stealing a sender address and a password. A connection apparatus 1 and a terminal 2 stores a plurality of passwords (or a plurality of password generation algorithms) and setting information, respectively. Times of the connection apparatus 1 and the terminal 2 are made to be synchronized with each other. The password (or the password generation algorithm) is modified based on the setting information with passage of time. Thus, the password is changed. The terminal 2 adds the password to a frame to be transmitted, and transmits to the connection apparatus 1. The connection apparatus 1 compares the password of the received frame with a password of the connection apparatus 1. When the passwords are in agreement with each other, the connection apparatus 1 permits the communication of the terminal 2.