A security device, a key device and a program protection system and method capable of coping with copy protection evading software or the modification of a board. A comparison is made between a security code unique to a game program which is obtained from a prestored key code and a security code obtained by generating a random number by using a game program stored in a storage device as an initial value. When they disagree, the operation of the function module is stopped. The copy protection is executed by hardware. By integrally forming the hardware and the function module, it is possible to cope with copy protection evading software or the modification of a board.