PURPOSE: To prevent a program used in a computer from being illegally copied.

CONSTITUTION: A protecting circuit 2 provided with a data operation circuit 20 having the same function as an arithmetic means 10 and a comparing means 14 on the side of a computer 1 and a comparing circuit 24 is connected between the computer 1 and a printer. A data string generated by a data string generating means 11 is given to them and is operates and it is discriminated whether their arithmetic results coincide with a preliminarily set value K or not by the comparing means 14 and the comparing circuit 24. When comparison results in coincidence, a signal Y is inverted from the low level to the high level by a signal transfer circuit 24 on the side of the computer 1. When comparison results in coincidence but the signal Y from the side of the protecting circuit 2 is not inverted or comparison does not result in coincidence but the signal Y is inverted, execution of a main program is stopped or interrupted by a program control means 15 on the side of the computer 1.