PURPOSE: To prevent a memory from being miswritten owing to a-noise, etc.

CONSTITUTION: When an address from a CPU 1 indicates a specific value, the memory 2 is allowed to be written and only in this writing-enabled state, the memory 2 is written. The specific value is a free address value which is not present in the address space of the memory 2. Consequently, miswriting is prevented through two steps, i.e., a 1st step wherein a write-enable register 4 is set according to the result obtained by decoding the address value by a decoder and a 2nd step wherein the memory 2 is allowed to be written with the output of an AND circuit 5, so the writing-enabled state is not easily entered and miswriting due to noise, etc., is seldom caused.