A program tamper detecting apparatus includes an external memory, an activation ROM and a CPU. The external memory stores a first code for program tamper detecting and a first program, wherein the first program is encrypted. The activation ROM stores a second program for decrypting the first program. The CPU is electrically connected to the external memory and the activation ROM. The CPU decrypts the first program by executing the second program to obtain the decrypted first program. The CPU detects tampering of the first program based on a comparison between the first code and a result of a predetermined operation executed on second codes of the decrypted first program.