To provide a printer with a copy guard function which makes a check under the control of a system side when a program is written so as to prevent data from being diverted to other printers and is able to operate the copy guard function having its operation changed according to the check result.

This printer has a function of obtaining a deciphering key from the operation part of the printer and an input means of an external computer via a communication means in the printer and a means for writing program into a storage means and is equipped with a program storage means which writes a program after the power source is turned on and stores the deciphering key obtained from the operation part in itself.