PURPOSE: To facilitate the rewriting of an instruction at the time of program debugging by permitting a program to mask the detection of a write protection error.

CONSTITUTION: A register 2 is normally set in a reset state, and the write protection error detected in an AND gate 102 is outputted from an AND gate 103 as it is. A write action is not executed and the exception processing of write protection occurs. A debugger program firstly sets the register 2 in the reset state, and rewrites a target instruction to a brake point instruction. Even if the gate 102 has detected the write protection error, the output 204 is masked by the gate 103, and the write action normally terminates. When rewriting into the brake point instruction terminates, the debugger program returns the register 2 in a set state, and executes the debugged program.