PURPOSE: To prevent the generation of illegal copying by including a command for finding out a respose from an original key in a program inputted from an original floppy to a computer, and when there is no response from the original key, stopping the advance of program reading.

CONSTITUTION: Software in the original key 20 includes a command for recognizing the existence of an AC wave, e.g. a square wave, and a command for recognizing frequency. At the time of starting the reading of a program, the computer 10 arrives at the command for checking the existence of a square wave and is requested to execute the command. When the square wave is inputted and its exisence is checked, the program goes to the succeeding command, frequency is judged, and when the frequency is included within a certain range, reading is continued. When the key 20 is not connected to an I/O port 24, the reading of the program is stopped. Namely the advance of the program is controlled by the combination of the existence of the square wave to be collating information and the original key provided with an original floppy 20 recording the frequency of the square wave and an oscillator.