A read only memory circuit for debugging and updating, the circuit includes read only memory, debug program memory, program counter, and compare and load unit. In the circuit, the compare and load unit detects instruction-read-memory-address from the program counter. If the instruction-read-memory-address is a predetermined main program address, the compare and load unit serves to transmit a debug address in the debug program memory to the program counter to update the original instruction-read-memory-address for debugging.